Impact of perinatal photoperiod on the chronotype of 11- to 18-year-olds in northern European Russia

The study investigates the effect of the month of birth and ambient light conditions at birth on sleep length and chronotype among residents of high latitudes. The authors surveyed 1172 persons (609 girls, 563 boys) age 11 to 18 yrs living in five villages and four towns located between 59.5°N and 67.6°N latitude. Survey participation was voluntary and anonymous. Sleep length and chronotype were assessed using the Munich chronotype questionnaire (MCTQ). The study showed the sleep length and chronotype of the children and adolescents depended on sex, age, type of settlement (town/village), and latitude of residence. Latitude exerted a stronger impact on sleep length and chronotype of children and adolescents living in villages than on those of their urban counterparts. Month of birth had no effect on sleep length and chronotype. There was a significant effect of the time of sunrise, sunset, and day length at birth on the chronotype of children and adolescents. A later chronotype was observed in the sample of young persons living above the Arctic Circle who were born during the polar day and polar night.     

Impact of Perinatal Photoperiod on the Chronotype of 11- to 18-Year-Olds in Northern European Russia

The study investigates the effect of the month of birth and ambient light conditions at birth on sleep length and chronotype among residents of high latitudes. The authors surveyed 1172 persons (609 girls, 563 boys) age 11 to 18 yrs living in five villages and four towns located between 59.5°N and 67.6°N latitude. Survey participation was voluntary and anonymous. Sleep length and chronotype were assessed using the Munich chronotype questionnaire (MCTQ). The study showed the sleep length and chronotype of the children and adolescents depended on sex, age, type of settlement (town/village), and latitude of residence. Latitude exerted a stronger impact on sleep length and chronotype of children and adolescents living in villages than on those of their urban counterparts. Month of birth had no effect on sleep length and chronotype. There was a significant effect of the time of sunrise, sunset, and day length at birth on the chronotype of children and adolescents. A later chronotype was observed in the sample of young persons living above the Arctic Circle who were born during the polar day and polar night. (Author correspondence: borisenkov@physiol.komisc.ru)     

Relevance of chronotype for eating patterns in adolescents

During adolescence, a shift from morningness to eveningness occurs, yet school continues to start early in the morning. Hence, adolescents are at risk for social jetlag, i.e. a discrepancy between biological and social timing. It remains to be determined whether chronotype associates with daily and daytime-specific eating patterns during this potentially critical period. Therefore, the aim of the present study was to investigate whether chronotype is decisive for daily eating patterns [total energy intake (TEI, kcal), total macronutrient intake (% of TEI), eating occasion frequency (n/day), meal frequency (n/day), snack frequency (n/day), duration of nightly fasting], or daytime-specific eating patterns [morning (before 11 am) energy intake (% of TEI), morning macronutrient intake (% of morning energy intake), regular breakfast skipping (no morning energy intake at least on 2 of 3?days, yes/no), evening (after 6 pm) energy intake (% of TEI), evening macronutrient intake (% of evening energy intake), regular dinner skipping (no evening energy intake at least on 2 of 3?days, yes/no)] in German adolescents. Chronotype was assessed by use of the Munich Chronotype Questionnaire and is defined as the midpoint of sleep corrected for sleep-debt accumulated over the workweek (the later the midpoint of sleep, the later the chronotype). A total of 223 participants (10–18?years) provided 346 questionnaires and concurrent 3-day weighed dietary records. Associations between chronotype and eating patterns were analyzed cross-sectionally using multivariable linear and logistic mixed-effects regression models. Adolescents with earlier and later chronotypes did not differ in their daily eating patterns. With respect to daytime-specific eating patterns, 1?h delay in chronotype was associated with 4.0 (95% CI 2.5–6.6) greater odds of regular breakfast skipping (p < 0.0001). In addition, later chronotype was associated with higher evening energy intake (p = 0.0009). In conclusion, our data show that a later chronotype among adolescents is associated with a shift of food consumption toward later times of the day. Hence, adolescents’ eating patterns appear to follow their internal clock rather than socially determined schedules.     

The pattern of entrainment of the human sleep-wake rhythm by the natural photoperiod in the north

Recently, it was shown that the sleep-wake rhythm of the inhabitants of the temperate zone is entrained to sun time. In the North, significant seasonal changes in the photoperiod may interfere with entrainment of the circadian system to sunlight. This investigation assessed the influence of photoperiod characteristics on the sleep length and sleep-wake rhythm of residents of high latitude. The study was conducted in four towns and six villages located between 59.5?N and 67.6?N latitude between the months of October and May from 2009 through 2011 and included 2822 subjects aged 10 to 97 yrs, 1621 of whom were females and 1201 males. The chronotype and sleep length of the subjects were assessed using the Munich Chronotype Questionnaire. The instructions for the questionnaire stressed the need to specify the sleep-wake schedule during the week preceding the date of completing the questionnaire. The study found that the length of sleep and the chronotype of the inhabitants surveyed in Northern European Russia depend on age, sex, type of settlement, and place of residence. The time of sunrise was a stronger predictor of sleep length and chronotype than the time of sunset and day length. A later chronotype and shorter sleep length were found for the subjects during the equinox (sunrise at 06:00 h) than under long-photoperiod conditions (sunrise at 04:00-05:00 h). During short-photoperiod conditions (sunrise at 07:00-10:00 h), no significant changes in the self-reported sleep-wake rhythm were found. The time of sunrise had the strongest impact on the sleep-wake rhythm of 30- to 97-yr-old persons. Sunrise had a stronger influence on chronotype and sleep length in January to May, when the days become longer, than in October to December, when the days become shorter. Age- and season-associated changes were found in the entrainment of the sleep-wake rhythm by photoperiod in the North.     

The Pattern of Entrainment of the Human Sleep-Wake Rhythm by the Natural Photoperiod in the North

Recently, it was shown that the sleep-wake rhythm of the inhabitants of the temperate zone is entrained to sun time. In the North, significant seasonal changes in the photoperiod may interfere with entrainment of the circadian system to sunlight. This investigation assessed the influence of photoperiod characteristics on the sleep length and sleep-wake rhythm of residents of high latitude. The study was conducted in four towns and six villages located between 59.5°N and 67.6°N latitude between the months of October and May from 2009 through 2011 and included 2822 subjects aged 10 to 97 yrs, 1621 of whom were females and 1201 males. The chronotype and sleep length of the subjects were assessed using the Munich Chronotype Questionnaire. The instructions for the questionnaire stressed the need to specify the sleep-wake schedule during the week preceding the date of completing the questionnaire. The study found that the length of sleep and the chronotype of the inhabitants surveyed in Northern European Russia depend on age, sex, type of settlement, and place of residence. The time of sunrise was a stronger predictor of sleep length and chronotype than the time of sunset and day length. A later chronotype and shorter sleep length were found for the subjects during the equinox (sunrise at 06:00?h) than under long-photoperiod conditions (sunrise at 04:00–05:00?h). During short-photoperiod conditions (sunrise at 07:00–10:00?h), no significant changes in the self-reported sleep-wake rhythm were found. The time of sunrise had the strongest impact on the sleep-wake rhythm of 30- to 97-yr-old persons. Sunrise had a stronger influence on chronotype and sleep length in January to May, when the days become longer, than in October to December, when the days become shorter. Age- and season-associated changes were found in the entrainment of the sleep-wake rhythm by photoperiod in the North. (Author correspondence: borisenkov@physiol.komisc.ru)     

Shift-specific associations between age,chronotype and sleep duration

The objective of this study was to examine the association of age with chronotype and sleep duration in day workers and rotating shift workers, including night shift work. Between October 2012 and February 2015, a cross-sectional study was conducted in a German chemical company. Using the “Munich ChronoType Questionnaire” (MCTQ), data about sleep onset and sleep offset during workdays and work-free days were retrieved and the chronotype was computed during regular voluntary occupational health check-ups. Associations between age and chronotype, as well as sleep duration, were assessed using linear regression analyses. Potential effect modification by the working time system was examined. Within the study period, 4,040 employees (82.3% and 17.7% were engaged in day work and rotating shift work, respectively) completed the questionnaire. Study participants were on average 41.8 years old (Min = 18.0, Max = 65.0, SD = 10.2) and predominantly male (75.4%). Mean chronotype and overall sleep duration was 03:22 (SD = 54 min) and 7.2 h (SD = 1.0 h) respectively. Older age was associated with earlier chronotype and reduced overall sleep duration in both day workers and rotating shift workers (p < 0.001 for all models). Compared to day workers, employees whom engaged in rotating shift work were later chronotypes and had overall a longer sleep duration. With older age, the difference between day and rotating shift workers regarding chronotype increased, while the difference regarding overall sleep duration decreased (p_interaction<0.005 for both models). This finding could indicate that both changes in circadian physiology and exposure to certain work schedules contribute to the age-related changes. Older rotating shift workers, with early chronotypes may have issues with night shifts, while day work and morning shifts may be best compatible to earlier chronotypes. Differences in sleep timing across age groups, might indicate that the same work hours will affect shift workers differently, dependent on their age, suggesting that more flexible and chronotype-adapted work hours could provide useful; especially for older employees. Sleep education in the form of courses and health campaigns could be a way to raise awareness of the importance of a healthy sleep pattern. This could be achieved by learning strategies to better adjust individual sleep patterns to work hours.     

Depression Scores Associate With Chronotype and Social Jetlag in a Rural Population

In public health, mood disorders are among the most important mental impairments. Patients with depressive episodes exhibit daily mood variations, abnormal patterns in sleep-wake behavior, and in the daily rhythms of several endocrine-metabolic parameters. Although the relationship between the sleep/circadian processes and mood disorders is poorly understood, clock-related therapies, such as light therapy, sleep deprivation, and rigid sleep schedules, have been shown to be effective treatments. Several studies investigated the relationship between circadian phenotype (chronotype) and depression. These focused mainly on urban populations and assessed diurnal preferences (Morningness-Eveningness score) rather than the actual timing of sleep and activity. Here, we used the Beck Depression Inventory (BDI) in an essentially rural population (N?=?4051), and investigated its relation to circadian phenotype (chronotype and social jetlag), assessed with the Munich Chronotype Questionnaire (MCTQ). In our study design, we (i) normalized both chronotype and BDI scores for age and sex (MSF_sas and BDI_as, respectively); (ii) calculated individual social jetlag (misalignment of the biological and social time); and (iii) investigated the relationship between circadian phenotypes and BDI scores in a population homogeneous in respect to culture, socioeconomic factors, and daily light exposure. A 15.65% (N?=?634) of the participants showed mild to severe depressive BDI scores. Late chronotypes had a higher BDI_as than intermediate and early types, which was independent of whether or not the participants were smokers. Both chronotype and BDI_as correlated positively with social jetlag. BDI_as was significantly higher in subjects with >2?h of social jetlag than in the rest of the population—again independent of smoking status. We also compared chronotype and social jetlag distributions between BDI categories (no symptoms, minimal symptoms, and mild to severe symptoms of depression) separately for men and women and for four age groups; specifically in the age group 31–40 yrs, subjects with mild to severe BDI scores were significantly later chronotypes and suffered from higher social jetlag. Our results indicate that misalignment of circadian and social time may be a risk factor for developing depression, especially in 31- to 40-yr-olds. These relationships should be further investigated in longitudinal studies to reveal if reduction of social jetlag should be part of prevention strategies. (Author correspondence: karla.allebrandt@med.uni-muenchen.de)     

Validity of the Japanese version of the Munich ChronoType Questionnaire

To assess circadian preference with a score, the Morningness-Eveningness Questionnaire (MEQ) has been used for more than 3 decades now. More recently, the Munich ChronoType Questionnaire (MCTQ) was developed: it asks for sleep-wake behavior on work and free days and uses the midpoint of sleep on free days (MSF), corrected for sleep debt accumulated during the work week as an indicator of chronotype (MSF_sc). In this study, we developed a Japanese version of the MCTQ by using a translation/back-translation approach including an examination of its semantic validity. In a subsequent questionnaire survey, 450 adult men and women completed the Japanese versions of the MCTQ and MEQ. Results showed that MEQ scores were significantly negatively correlated with mid-sleep parameters assessed by the MCTQ, on both, work and free days, as well as with the chronotype measure MSF_sc (r?=??0.580 to ?0.652, all p?<?0.001). As in the original German version, the strongest correlation was observed between MEQ score and MSF. A physiological validation study using dim light melatonin onset as a circadian phase marker (N?=?37) showed a high correlation between chronotype as assessed with the MSF_sc (r?=?0.542, p?<?0.001), and less so for MEQ score (r?=??0.402, p?=?0.055). These results demonstrate the validity of the Japanese MCTQ and provide further support of the adequacy of the MCTQ as a chronotype measure.     

Depression scores associate with chronotype and social jetlag in a rural population

In public health, mood disorders are among the most important mental impairments. Patients with depressive episodes exhibit daily mood variations, abnormal patterns in sleep-wake behavior, and in the daily rhythms of several endocrine-metabolic parameters. Although the relationship between the sleep/circadian processes and mood disorders is poorly understood, clock-related therapies, such as light therapy, sleep deprivation, and rigid sleep schedules, have been shown to be effective treatments. Several studies investigated the relationship between circadian phenotype (chronotype) and depression. These focused mainly on urban populations and assessed diurnal preferences (Morningness-Eveningness score) rather than the actual timing of sleep and activity. Here, we used the Beck Depression Inventory (BDI) in an essentially rural population (N?=?4051), and investigated its relation to circadian phenotype (chronotype and social jetlag), assessed with the Munich Chronotype Questionnaire (MCTQ). In our study design, we (i) normalized both chronotype and BDI scores for age and sex (MSF(sas) and BDI(as), respectively); (ii) calculated individual social jetlag (misalignment of the biological and social time); and (iii) investigated the relationship between circadian phenotypes and BDI scores in a population homogeneous in respect to culture, socioeconomic factors, and daily light exposure. A 15.65% (N?=?634) of the participants showed mild to severe depressive BDI scores. Late chronotypes had a higher BDI(as) than intermediate and early types, which was independent of whether or not the participants were smokers. Both chronotype and BDI(as) correlated positively with social jetlag. BDI(as) was significantly higher in subjects with >2?h of social jetlag than in the rest of the population?again independent of smoking status. We also compared chronotype and social jetlag distributions between BDI categories (no symptoms, minimal symptoms, and mild to severe symptoms of depression) separately for men and women and for four age groups; specifically in the age group 31?40 yrs, subjects with mild to severe BDI scores were significantly later chronotypes and suffered from higher social jetlag. Our results indicate that misalignment of circadian and social time may be a risk factor for developing depression, especially in 31- to 40-yr-olds. These relationships should be further investigated in longitudinal studies to reveal if reduction of social jetlag should be part of prevention strategies. (Author correspondence: karla.allebrandt@med.uni-muenchen.de ).     

Chronotype and postmenopausal breast cancer risk among women in the California Teachers Study

ABSTRACT

Chronotype is the behavioral manifestation of an individual’s underlying circadian rhythm, generally characterized by one’s propensity to sleep at a particular time during the 24 hour cycle. Evening chronotypes (“night owls”) generally suffer from worse physical and mental health compared to morning chronotypes (“morning larks”) – for reasons that have yet to be explained. One hypothesis is that evening chronotypes may be more susceptible to circadian disruption, a condition where the coordinated timing of biologic processes breaks down. The role of chronotype as an independent or modifying risk factor for cancer has not been widely explored. The objective of the current study was to evaluate the risk of breast cancer associated with chronotype in a case-control study nested within the California Teachers Study (CTS) cohort. The study population consisted of 39686 post-menopausal CTS participants who provided information on chronotype by completing a questionnaire in 2012–2013. 2719 cases of primary invasive breast cancer diagnosed from 1995/1996 through completion of the chronotype questionnaire were identified by linkage of the CTS to the California Cancer Registry. 36967 CTS participants who had remained cancer-free during this same time period served as controls. Chronotype was ascertained by responses to an abbreviated version of the Horne-Ostberg Morningness-Eveningness Questionnaire (MEQ) and was characterized into five categories: definite morning, more morning than evening, neither morning or evening, more evening than morning, definite evening. Multivariable unconditional logistic regression analyses were performed to estimate the odds ratios (ORs) and 95% confidence intervals (95% CIs) for each of the chronotypes, adjusted for established breast cancer risk factors. Compared to definite morning types, definite evening types had an increased risk of breast cancer with elevated ORs that were statistically significant in both the crude (OR = 1.24, 95% CI: 1.10–1.40) and fully-adjusted models (OR = 1.20, 95% CI: 1.06–1.35). The risk estimates in the fully-adjusted model for all other chronotypes did not significantly differ from one. These results suggest that evening chronotype may be an independent risk factor for breast cancer among a population of women who are not known to have engaged in any substantial night shift work. Further research in other populations of non-shift workers is warranted.     

Circadian clock properties of fruit flies Drosophila melanogaster exhibiting early and late emergence chronotypes

The role of circadian clocks in timing daily behaviors is widely acknowledged, and while empirical evidence suggests that clock period is correlated with the preferred phase of a rhythmic behavior (chronotype), other clock properties have also been hypothesized to underlie chronotype variation. Here, we report that fruit fly Drosophila melanogaster populations exhibiting evening emergence chronotype (late) are characterized by higher incidence of behavioral arrhythmicity in constant dim light, wider range of entrainment, reduced rates of re-entrainment to simulated jet-lag and higher amplitude of both entrained and free-running rhythms as compared to those exhibiting morning emergence chronotype (early). Our results thus highlight the role of circadian clock properties such as zeitgeber sensitivity, amplitude and coupling in driving chronotype variation.     

Individual differences in chronotypes associated with academic performance among Chilean University students

A chronotype is an individual trait that determines circadian rhythm (dark/light cycle) characteristics, associated with bedtime, waking, and other daily activities. A chronotype is classified as morning, intermediate, and evening. The objective is to associate chronotypes with academic performance in university students. A cross-sectional study was performed to evaluate the chronotype of university students (n = 703) by Horne-Ostberg questionnaire and associated with academic performance. The group with higher GPAs had higher chronotype scores (p = 0.002). Morning and intermediate chronotypes exhibited better academic performance; however, more studies are necessary to determine the underlying causes, which could influence cognitive aspects.     

The stimulating effect of bright light on physical performance depends on internal time

The human circadian clock regulates the daily timing of sleep, alertness and performance and is synchronized to the 24-h day by the environmental light-dark cycle. Bright light exposure has been shown to positively affect sleepiness and alertness, yet little is known about its effects on physical performance, especially in relation to chronotype. We, therefore, exposed 43 male participants (mean age 24.5 yrs ± SD 2.3 yrs) in a randomized crossover study to 160 minutes of bright (BL: ≈ 4.420 lx) and dim light (DL: ≈ 230 lx). During the last 40 minutes of these exposures, participants performed a bicycle ergometer test. Time-of-day of the exercise sessions did not differ between the BL and DL condition. Chronotype (MSF(sc), mid-sleep time on free days corrected for oversleep due to sleep debt on workdays) was assessed by the Munich ChronoType Questionnaire (MCTQ). Total work was significantly higher in BL (median 548.4 kJ, min 411.82 kJ, max 875.20 kJ) than in DL (median 521.5 kJ, min 384.33 kJ, max 861.23 kJ) (p = 0.004) going along with increased exhaustion levels in BL (blood lactate (+12.7%, p = 0.009), heart rate (+1.8%, p = 0.031), and Borg scale ratings (+2.6%, p = 0.005)) in all participants. The differences between total work levels in BL and DL were significantly higher (p = 0.004) if participants were tested at a respectively later time point after their individual mid-sleep (chronotype). These novel results demonstrate, that timed BL exposure enhances physical performance with concomitant increase in individual strain, and is related not only to local (external) time, but also to an individual's internal time.     

Daily activity and light exposure levels for five species of lemurs at the duke lemur center

Light is the primary synchronizer of all biological rhythms, yet little is known about the role of the 24‐hour luminous environment on nonhuman primate circadian patterns, making it difficult to understand the photic niche of the ancestral primate. Here we present the first data on proximate light–dark exposure and activity–rest patterns in free‐ranging nonhuman primates. Four individuals each of five species of lemurs at the Duke Lemur Center (Eulemur mongoz, Lemur catta, Propithecus coquereli, Varecia rubra, and Varecia variegata variegata) were fitted with a Daysimeter‐D pendant that contained light and accelerometer sensors. Our results reveal common as well as species‐specific light exposure and behavior patterns. As expected, all five species were more active between sunrise and sunset. All five species demonstrated an anticipatory increase in their pre‐sunrise activity that peaked at sunrise with all but V. rubra showing a reduction within an hour. All five species reduced activity during mid‐day. Four of the five stayed active after sunset, but P. coquereli began reducing their activity about 2 hours before sunset. Other subtle differences in the recorded light exposure and activity patterns suggest species‐specific photic niches and behaviors. The eventual application of the Daysimeter‐D in the wild may help to better understand the adaptive evolution of ancestral primates. Am J Phys Anthropol 153:68–77, 2014. © 2013 Wiley Periodicals, Inc.     

Zeitgebers and their association with rest-activity patterns

Zeitgebers such as light, eating and physical activity provide input to the circadian clock. Chronic circadian misalignment is associated with significant adverse health effects. An improved understanding of the impact of the timing of zeitgebers on the stability of 24-hour rest-activity rhythm in free-living settings may identify behavioural and environmental intervention targets. A total of 133 healthy adults, aged 21–60 years, wore a wrist actigraph for 7 consecutive days. We applied a non-parametric analysis to activity counts to derive rest-activity patterns. We administered a questionnaire through a smartphone app to collect self-reported timing of light exposure, eating episodes and physical activity. To assess the relationship between timing exposures (first and last exposure to outdoor light, first exposure to indoor light, last eating episode, first eating episode, morning physical activity proportion, evening physical activity proportion) and rest-activity or sleep outcomes (bedtimes, total sleep time, inter-daily stability, intra-daily variability, L5 and M10 midpoint), we first calculated Spearman correlations, using the false discovery rate method to control for multiple comparisons. From those significant associations, we then fit regression models adjusting for age, sex, race, household income, education level, study site, body mass index, as well as physical activity. Finally, we tested for interaction between chronotype and each timing-related exposure and stratified the analysis by morning type. All zeitgebers, except for evening physical activity proportion, were correlated with at least four of the seven sleep and rest-activity outcomes. In adjusted analysis, later timing of first (after 6:30 to 7:45 AM versus earlier) and last exposure to indoor light (after 11:00 PM versus earlier) and first (after 7:45–9:45 AM versus earlier) and last eating episode (after 8:00–09:00 PM versus earlier) were associated with a shift of 0.60–1.39 hours to later bedtimes, M10 and L5 midpoints (i.e. timing of peak activities or inactivities). Later timing of first exposure to outdoor light (after 09:30 AM versus earlier) was also associated with 0.51 (95% CI: 0.19 to 0.83) hours longer total sleep time. Higher morning physical activity proportion (> 33%) was associated with 0.95 (95% CI: ?1.38 to ?0.53) hours earlier in-bed time and 0.69 (95% CI: ?1.14 to ?0.24) hours earlier out-of-bed time, 0.92 (95% CI: ?1.41 to ?0.42) hours earlier M10 and 0.96 (95% CI: ?1.42 to ?0.49) min earlier L5 midpoint. The results did not change substantially with further adjustment for total activity. There was a significant interaction between morning chronotype and first eating episode with rest-activity patterns (p < 0.05), with first eating episode associating with timing of activities only in non-morning type adults. Timing of zeitgebers was associated with sleep and rest-activity patterns, including bedtimes, L5 and M10 midpoint. Future research should evaluate the impact of manipulating zeitgebers on both circadian rhythms and health outcomes.     

Chronotype and energy intake timing in relation to changes in anthropometrics: a 7-year follow-up study in adults

Individuals with a later preference for the daily activities (evening types) tend to have unhealthier behaviors, which could increase their risk for obesity when compared those with an earlier preference (morning types). Furthermore, later food intake timing, another behavior more characteristic of evening types, has been associated with obesity. However, chronotype differences in the long-term weight change and the role of chronotype in the association between energy intake timing and obesity risk are not clear. To study this we first examined the independent associations of chronotype and energy intake timing with anthropometric changes and then whether chronotype modified the association between energy intake timing and obesity risk. Our data included 1097 Finns from DILGOM (DIetary Lifestyle and Genetic Determinants of Obesity and Metabolic syndrome) 2007 (baseline) and 2014 (follow-up) and from Findiet 2007. Chronotype was assessed with a shortened version of Horne and Östberg’s morningness–eveningness questionnaire. Energy intake timing (as percentages of the total energy intake in the morning/evening) was assessed with 48-h dietary recalls. Weight, body mass index (BMI), and waist circumference were based on measured and self-reported values. Analysis of co-variance and multivariable logistic regression models were used for statistical analyses. Evening typed women had greater weight gain (+ 2.3 kg vs. + 0.3 kg, P = 0.016) and increase in BMI (0.7 kg/m² vs. ?0.1 kg/m², P = 0.024) than morning typed women. After excluding participants with depression, these associations attenuated to non-significant. Compared to participants whose energy intake was proportionally lowest during evening, those with proportionally highest energy intake during evening were more likely with obesity (BMI≥ 30 kg/m²) after follow-up (OR 1.97, 95% CI 1.21–3.21, Ptrend = 0.042). Participants’ chronotype did not modify this association (Pinteract = 0.95). In conclusion, our findings indicated that evening energy intake may play a role in obesity regardless of the chronotype. Furthermore, evening typed women were more prone to increases in their anthropometrics, which seem to be at least partly explained by depression. Further studies of this topic are warranted.     

Circadian phase,circadian period and chronotype are reproducible over months

The timing of the circadian clock, circadian period and chronotype varies among individuals. To date, not much is known about how these parameters vary over time in an individual. We performed an analysis of the following five common circadian clock and chronotype measures: 1) the dim light melatonin onset (DLMO, a measure of circadian phase), 2) phase angle of entrainment (the phase the circadian clock assumes within the 24-h day, measured here as the interval between DLMO and bedtime/dark onset), 3) free-running circadian period (tau) from an ultradian forced desynchrony protocol (tau influences circadian phase and phase angle of entrainment), 4) mid-sleep on work-free days (MSF from the Munich ChronoType Questionnaire; MCTQ) and 5) the score from the Morningness–Eveningness Questionnaire (MEQ). The first three are objective physiological measures, and the last two are measures of chronotype obtained from questionnaires. These data were collected from 18 individuals (10 men, eight women, ages 21–44 years) who participated in two studies with identical protocols for the first 10 days. We show how much these circadian rhythm and chronotype measures changed from the first to the second study. The time between the two studies ranged from 9 months to almost 3 years, depending on the individual. Since the full experiment required living in the laboratory for 14 days, participants were unemployed, had part-time jobs or were freelance workers with flexible hours. Thus, they did not have many constraints on their sleep schedules before the studies. The DLMO was measured on the first night in the lab, after free-sleeping at home and also after sleeping in the lab on fixed 8-h sleep schedules (loosely tailored to their sleep times before entering the laboratory) for four nights. Graphs with lines of unity (when the value from the first study is identical to the value from the second study) showed how much each variable changed from the first to the second study. The DLMO did not change more than 2 h from the first to the second study, except for two participants whose sleep schedules changed the most between studies, a change in sleep times of 3 h. Phase angle did not change by more than 2 h regardless of changes in the sleep schedule. Circadian period did not change more than 0.2 h, except for one participant. MSF did not change more than 1 h, except for two participants. MEQ did not change more than 10 points and the categories (e.g. M-type) did not change. Pearson’s correlations for the DLMO between the first and second studies increased after participants slept in the lab on their individually timed fixed 8-h sleep schedules for four nights. A longer time between the two studies did not increase the difference between any of the variables from the first to the second study. This analysis shows that the circadian clock and chronotype measures were fairly reproducible, even after many months between the two studies.     

Sleep quality and methylation status of core circadian rhythm genes among nurses and midwives

Poor sleep quality or sleep restriction is associated with sleepiness and concentration problems. Moreover, chronic sleep restriction may affect metabolism, hormone secretion patterns and inflammatory responses. Limited recent reports suggest a potential link between sleep deprivation and epigenetic effects such as changes in DNA methylation profiles. The aim of the present study was to assess the potential association between poor sleep quality or sleep duration and the levels of 5-methylcytosine in the promoter regions of PER1, PER2, PER3, BMAL1, CLOCK, CRY1 CRY2 and NPAS2 genes, taking into account rotating night work and chronotype as potential confounders or modifiers. A cross-sectional study was conducted on 710 nurses and midwives (347 working on rotating nights and 363 working only during the day) aged 40–60 years. Data from in-person interviews about sleep quality, chronotype and potential confounders were used. Sleep quality and chronotype were assessed using Pittsburgh Sleep Quality Questionnaire (PSQI) and Morningness–Eveningness Questionnaire (MEQ), respectively. Morning blood samples were collected. The methylation status of the circadian rhythm genes was determined via quantitative methylation-specific real-time PCR assays (qMSP) reactions using DNA samples derived from leucocytes. The proportional odds regression model was fitted to quantify the relationship between methylation index (MI) as the dependent variable and sleep quality or sleep duration as the explanatory variable. Analyses were carried out for the total population as well as for subgroups of women stratified by the current system of work (rotating night shift/day work) and chronotype (morning type/intermediate type/evening type). A potential modifying effect of the system of work or the chronotype was examined using the likelihood ratio test. No significant findings were observed in the total study population. Subgroup analyses revealed two statistically significant associations between a shorter sleep duration and 1) methylation level in PER2 among day workers, especially those with the morning chronotype (OR = 2.31, 95%CI:1.24–4.33), and 2) methylation level in CRY2 among subjects with the intermediate chronotype, particularly among day workers (OR = 0.52, 95%CI:0.28–0.96). The study results demonstrated a positive association between average sleep duration of less than 6 hours and the methylation level of PER2 among morning chronotype subjects, and an inverse association for CRY2 among intermediate chronotype subjects, but only among day workers. Both the system of work and the chronotype turned out to be important confounders and modifiers in a number of analyses, making it necessary to consider them as potential covariates in future research on sleep deficiency outcomes. Further studies are warranted to explore this under-investigated topic.     

LIGHT INTENSITY AND THE TIMING OF DAILY ACTIVITY OF FINCHES (FRINGILLIDAE)

The prevailing general theory of entrainment of endogenous circadian rhythms explains variation in the timing of activity with season, latitude and weather conditions on the basis of the dependence of spontaneous frequency of the rhythm on the intensity of constant illumination. In an experiment designed to test the validity of this theory, Greenfinches and chaffinches were exposed to the natural light-dark cycle alternately in full illumination and in reduced daylight. Contrary to prediction by current hypotheses for these species, in which the spontaneous frequency decreases with decreasing light intensity, their midpoint of daily activity shifted forward when the intensity of natural daylight was reduced. This result refutes the hypothesis that light affects the timing of activity in nature in a manner predictable from its effect in constant conditions.
Both species of finches have a bimodal daily distribution of perch-hopping activity. The morning peak and evening peak of activity are of about equal strength in winter, but the morning peak increases sharply in the reproductive season. The possibility that the two peaks represent two pacemaker systems, under differential endocrine control, is discussed.
The precision in the timing of daily onset and end of activity is positively correlated with the estimated average rate of change of light intensity at these times of day. Thus, day-to-day variations are markedly increased in reduced daylight when activity begins late after sunrise and terminates well before sunset.     

Effects of Temperature on Circadian Clock and Chronotype: An Experimental Study on a Passerine Bird

Daily schedules of many organisms, including birds, are thought to affect fitness. Timing in birds is based on circadian clocks that have a heritable period length, but fitness consequences for individuals in natural environments depend on the scheduling of entrained clocks. This chronotype, i.e., timing of an individual relative to a zeitgeber, results from interactions between the endogenous circadian clock and environmental factors, including light conditions and ambient temperature. To understand contributions of these factors to timing, we studied daily activity patterns of a captive songbird, the great tit (Parus major), under different temperature and light conditions. Birds were kept in a light (L)-dark (D) cycle (12.5?L:11.5 D) at either 8°C or 18°C with ad libitum access to food and water. We assessed chronotype and subsequently tested birds at the same temperature under constant dim light (LL_dim) to determine period length of their circadian clock. Thermal conditions were then reversed so that period length was measured under both temperatures. We found that under constant dim light conditions individuals lengthened their free-running period at higher temperatures by 5.7?±?2.1?min (p?=?.002). Under LD, birds kept at 18°C started activity later and terminated it much earlier in the day than those kept under 8°C. Overall, chronotype was slightly earlier under higher temperature, and duration of activity was shorter. Furthermore, individuals timed their activities consistently on different days under LD and over the two test series under LL_dim (repeatability from .38 to .60). Surprisingly, period length and chronotype did not show the correlation that had been previously found in other avian species. Our study shows that body clocks of birds are precise and repeatable, but are, nonetheless, affected by ambient temperature. (Author correspondence: marina.lehmann@uni-konstanz.de)