Identification of circadian rhythms in cognitive inhibition and flexibility using a Stroop task

The objective of this study was to identify circadian rhythms in cognitive inhibition and flexibility using a Stroop task. Eight undergraduate student volunteers (age = 17.75 ± 0.46 years; 7 females), participated in this study. Participants were recorded in a constant routine protocol for 29 h. Body temperature was recorded every minute, and every 100 min participants completed scales of sleepiness and tiredness and responded to a Stroop task with shifting criteria. This task includes a chart with 48 colors printed with incongruent names. A random half of the words had a point on the left. Participants were instructed to do four actions: (i) read all the words; (ii) name the color in which each word was printed; (iii) read the words marked with a point and name the colors of the unmarked words; and (iv) name the colors of the words marked with a point and read the unmarked words. The time and errors in naming the colors were considered inhibition indices, while performance time and errors upon shifting criteria were considered flexibility indices. Circadian variations were found in inhibition (color-naming) and flexibility (shifting criteria). These cognitive processes also showed decay with time on task. In conclusion, there were circadian variations in two components of executive functions: inhibition and flexibility, modulated by sustained attention (time on task). These results may explain the difficulties experienced at night in problem-solving that requires adjustment of behavior in accordance with environmental demands.

     

Circadian and homeostatic modulation of the attentional blink

An important property of attention is the limitation to process new information after responding to a stimulus. This property of attention can be evaluated by the Attentional Blink (AB), a phenomenon that consists of a failure to detect the second of two targets when the interval between them is 200–500 ms. The aim of the present work is to determine the possible existence of time awake (homeostatic changes) and time of day (circadian rhythm) variations in the AB. Eighteen undergraduate students, 11 men and 7 women, age = 18.06 ± 1.16 years, participated voluntarily in this research. They were recorded in a constant routine protocol during 29 h, in which rectal temperature was recorded every minute, while subjective sleepiness and responses to a Rapid Serial Visual Presentation (RSVP) task, to measure the AB, were recorded every hour. Homeostatic and circadian variations in all parameters of the RSVP task were observed, including changes in the capacity to process a new stimulus (Target 1 accuracy), a second stimulus occurring in a short interval after the first (Target 2 accuracy at lag 2, 200 ms) and to process another successive independent stimulus (Target 2 accuracy at lag 8, 800 ms). The acrophase of these parameters occurred with a phase delay of 2 h compared to the circadian rhythm of rectal temperature. The AB magnitude, an index of the AB, showed a decline with time awake, but no variations with time of day. In conclusion, there are homeostatic and circadian variations in the capacity to process any incoming information, especially in tasks with brief duration stimuli presented at a high frequency.     

Circadian rhythms in phonological and visuospatial storage components of working memory

Working memory is a basic cognitive process that temporarily maintains the information necessary for the performance of many complex tasks such as reading comprehension, learning and reasoning. Working memory includes two storage components: phonological and visuospatial, and a central executive control. The objective of this study was to identify possible circadian rhythms in phonological and visuospatial storage components of working memory using a constant routine protocol. Participants were eight female undergraduate students, aged 17.5±0.93, range = 16 - 19 years old. They were recorded in the laboratory in a constant routine protocol during 30 h. Rectal temperature was recorded every minute; subjective sleepiness and tiredness, as well as phonological and visuospatial working memory tasks, were assessed each hour. There were circadian variations in correct responses in phonological and visuospatial working memory tasks. Cross-correlation analysis showed a 1-h phase delay of the phonological storage component and a 3-h phase delay of the visuospatial storage component with respect to rectal temperature. This result may explain the changes in the performance of many complex tasks during the day.     

Evening physical activity alters wrist temperature circadian rhythmicity

The adequate time to perform physical activity (PA) to maintain optimal circadian system health has not been defined. We studied the influence of morning and evening PA on circadian rhythmicity in 16 women with wrist temperature (WT). Participants performed controlled PA (45?min continuous-running) during 7 days in the morning (MPA) and evening (EPA) and results were compared with a no-exercise-week (C). EPA was characterized by a lower amplitude (evening: 0.028?±?0.01?°C versus control: 0.038?±?0.016?°C; p?<?0.05) less pronounced second-harmonic (power) (evening: 0.41?±?0.47 versus morning: 1.04?±?0.59); and achrophase delay (evening: 06:35?±?02:14?h versus morning: 04:51?±?01:11?h; p?<?0.05) as compared to MPA and C. Performing PA in the late evening might not be as beneficial as in the morning.     

Daily rhythm of some haematological parameters in Holstein bovine maintained under natural conditions in southern hemisfere

The aim of this study was to evaluate the daily rhythms of haematological responses in bovines under different environmental conditions and to investigate the differences between bulls and cows. Twelve Holstein bovines were divided into two groups according to their gender: Group A (6 cows, 2 years, 500 ± 25 kg), and Group B (6 bulls, 2 years, 520 ± 25 kg). Environmental temperature and relative humidity were recorded and the temperature humidity index (THI) was calculated. Blood samples were collected every 3 h over 24 h period, every three months (winter, spring, summer, autumn) and analysed for haematological parameters. A significant effect of season (p < 0.01) and of gender (p < 0.05) on all studied parameters, and a circadian rhythm of RBC, Hb and Hct in all periods were found. These results may enhance the knowledge of haematological changes in cattles under natural environmental conditions, and their adaptation to the temporisations imposed by the environment.     

Twenty-four-hour variation of vestibular function in young and elderly adults

ABSTRACT

Animal and human studies demonstrate anatomical and functional links between the vestibular nuclei and the circadian timing system. This promotes the hypothesis of a circadian rhythm of vestibular function. The objective of this study was to evaluate the vestibular function through the vestibulo-ocular reflex using a rotatory chair at different times of the day to assess circadian rhythmicity of vestibular function. Two identical studies evaluating temporal variation of the vestibulo-ocular reflex (VOR) were performed, the first in young adults (age: 22.4 ± 1.5 y), and the second in older adults (70.7 ± 4.7 y). The slow phase velocity and time constant of the VOR were evaluated in six separate test sessions, i.e., 02:00, 06:00, 10:00, 14:00, 18:00, and 22:00 h. In both studies, markers of circadian rhythmicity (temperature, fatigue, and sleepiness) displayed expected usual temporal variation. In young adults, the time constant of the VOR showed variation throughout the day (p < .005), being maximum 12:25 h (06:00 h test session) before the acrophase of temperature circadian rhythm. In older adults, the slow phase velocity and time constant also displayed temporal variation (p < .05). Maximum values were recorded at 10:35 h (06:00 h test session) before the acrophase of temperature circadian rhythm. The present study demonstrates that vestibular function is not constant throughout the day. The implication of the temporal variation in vestibular system in equilibrium potentially exposes the elderly, in particular, to differential risk during the 24 h of losing balance and falling.     

Different behavior of body temperature and total locomotor activity daily rhythms during light/dark cycle in stabled Oryctolagus cuniculus

In order to investigate the potential causal link between the rhythm of activity and body temperature, we simultaneously recorded rectal temperature and total locomotor activity in five clinically healthy female rabbits (blue Vienna breed), 12 week old and mean body weight 2.7 ± 0.3. Animals were housed in individual cages (90?×?50?×?35 cm) under natural 12/12 light/dark cycle. Total locomotor activity was monitored for 15 days by an activity data logger. On day 1, 5, 10, and 15 rectal temperature was recorded every 2 h for a 24-h period. Application of single cosinor method showed a nocturnal daily rhythm of rectal temperature with a range of oscillation of about 1 °C, acrophase after dusk and low robustness value. The daily rhythm of locomotor activity showed its acrophase in the middle of the scotophase and a high robustness value. This information improves the knowledge available on the circadian biology of rabbits useful in the evaluation of physiology of this species.     

Effect of Time of Day on Aerobic Contribution to the 30‐s Wingate Test Performance

The purpose of this study was to evaluate the effects of time of day on aerobic contribution during high‐intensity exercise. A group of 11 male physical education students performed a Wingate test against a resistance of 0.087 kg · kg^?1 body mass. Two different times of day were chosen, corresponding to the minimum (06:00 h) and the maximum (18:00 h) levels of power. Oxygen uptake (V˙O₂) was recorded breath by breath during the test (30 sec). Blood lactate concentrations were measured at rest, just after the Wingate test, and again 5 min later. Oral temperature was measured before each test and on six separate occasions at 02:00, 06:00, 10:00, 14:00, 18:00, and 22:00 h. A significant circadian rhythm was found in body temperature with a circadian acrophase at 18:16±00:25 h as determined by cosinor analysis. Peak power (P_peak), mean power (P_mean), total work done, and V˙O₂ increased significantly from morning to afternoon during the Wingate Test. As a consequence, aerobic contribution recorded during the test increased from morning to afternoon. However, no difference in blood lactate concentrations was observed from morning to afternoon. Furthermore, power decrease was greater in the morning than afternoon. Altogether, these results indicate that the time‐of‐day effect on performances during the Wingate test is mainly due to better aerobic participation in energy production during the test in the afternoon than in the morning.     

Administration-Time Differences in the Pharmacokinetics of Gentamicin Intravenously Delivered to Human Beings

Administration-time differences of gentamicin pharmacokinetics were studied by crossover design after a single intravenous administration of gentamicin (80 mg) to 10 human subjects at 09:00 (morning time) and 22:00 (nighttime). The profiles of serum gentamicin concentration showed a significant statistical difference between 09:00 and 22:00, suggesting circadian variations of pharmacokinetic behaviors. A significant circadian rhythm of pharmacokinetic parameters as a function of time of day was noted in human subjects, showing lower total body clearance Cl_t and higher serum area under the curve (AUC) when given at nighttime. The half-life t_1/2 was shorter in the morning (2.82h ± 0.43h) when compared to the nighttime (2.97h ± 0.36h), but the difference was not statistically significant. The AUC was significantly greater in the morning (23.4 ± 3.84 μg-h/mL) than that in the nighttime (26.3 ± 5.79 μg-h/mL) (p<. 05), most likely because the Cl_t, was significantly higher when gentamicin was given in the morning (3.51 ± 0.57 L/h) versus in the nighttime (3.18 ± 0.65 L/h). Although the volume of distribution V_d decreased when given at nighttime, it was independent of the dosing time. From this study, there was an administration-time difference of gentamicin pharmacokinetics in human beings. The optimized dosing regimen of gentamicin can be decided by considering circadian rhythm and rest-activity routine so that minimized toxicity and effective therapy are established for patients. The current findings also can be applied to other drugs with circadian rhythms of pharmacokinetics and narrow therapeutic windows in clinical chronotherapeutics.     

Effect of sport practice and warm-up duration on the morning–evening difference in anaerobic exercise performance and perceptual responses to it

This study was designed to assess the effect of sport practice and warm-up duration on the morning–evening differences in muscle power and fatigue during performance of anaerobic exercise and perceptual responses to it. Twenty-two male physical education students – twelve trained (TG) (21.51 ± 1.25 years, 182.17 ± 4.37 cm and 82.88 ± 11.23 kg) and ten untrained (NTG) (23.89 ± 3.17 years, 176.8 ± 2.2 cm and 82.24 ± 8.43 kg) – participated in a crossover randomized study. They were asked to perform a 30-s Wingate test during six experimental sessions (three at 08:00 and three at 18:00 h) after different active warm-up (AWU) durations (5 min, 15 min, or 20 min). The AWU consisted of pedaling at a constant pace of 60 rpm against 50% of maximal aerobic power. Rate of perceived exertion (RPE) was recorded after the AWU and again immediately after the Wingate test. Heart rate and temperature (T) were recorded during each session at rest, after AWU and immediately at the end of the Wingate test. During the Wingate test, peak power (PP), mean power (MP), and the fatigue index were recorded. While the RPE estimations were higher in NTG, no time-of-day effect was recorded in either experimental group (morning or evening). T, PP, and MP were higher in the afternoon than in the morning (p < 0.001 for PP and MP; p < 0.05 for T). Similarly, PP and MP during the Wingate test were significantly higher in the TG than in the NTG (p < 0.001). The morning–evening difference of PP and MP was affected by AWU duration; AWU₁₅ was best in the morning for improving lower limb power for both the TG and NTG, whereas reducing this period to 5 min in the evening was appropriate for TG only.     

Diurnal variation in stroke parameters and motor organization in front-crawl swimmers

The aim of the present study was to examine the effects of time of day on stroke parameters and motor organization in front-crawl swimmers. In a randomized order, fourteen regional swimmers (age: 18.7 ± 1.6 years) performed maximal front crawls over 12.5 m during two experimental sessions; the morning sessions were conducted between 07:00 and 09:00 h and the evening experiments were conducted between 17:00 and 19:00 h. Stroke parameters (swim velocity, stroke rate [SR], and stroke length), motor organization (arm stroke phases and arm coordination) were calculated from aerial and underwater side-view cameras. Arm coordination was quantified in terms of an index of coordination (Idc). Results showed that oral temperature was significantly higher in the evening 36.8 ± 0.2 °C than in the morning 36.1 ± 0.2 °C (p < 0.001), with a morning–evening difference of ?0.7 ± 0.1 °C. Performance was also higher in the evening (7.4 ± 0.6 s) than in the morning (8.0 ± 0.8 s) (p < 0.001), with a morning–evening difference of 0.55 ± 0.30 s. Likewise, values of swim velocity and SR were higher in the evening than in the morning (p < 0.001) with morning–evening differences of ?0.10 ± 0.04 m s^?1 and ?3.99 ± 2.91 cycles min^?1, respectively. Percentage Idc increased significantly (p < 0.01) between the morning (?5.1 ± 6.5%) and evening (?1.6 ± 7.0%). It is concluded that maximal swimming trials are performed better in the evening than the morning, and that this might be explained by better stroke parameters and motor organization at this time.     

Alertness and psychomotor performance levels of marine pilots on an irregular work roster

ABSTRACT

Fatigue is recognized as an important safety concern in the transportation industry. In this study, our goal was to investigate how circadian and sleep–wake dependent factors influence St-Lawrence River pilots’ sleep–wake cycle, alertness and psychomotor performance levels at work. A total of 18 male St-Lawrence River ship pilots were recruited to a 16–21-day field study. Pilots’ chronotype, sleepiness and insomnia levels were documented using standardized questionnaires. Their sleep–wake cycle was documented by a sleep–wake log and wrist-worn activity monitoring. Subjective alertness and objective psychomotor performances were assessed ~5×/day for each work and rest day. Ship transits were distributed throughout the 24-h day and lasted on average (± SEM) 5.93 ± 0.67 h. Main sleep periods occurred mainly at night, and objectively lasted 6.04 ± 1.02 h before work days. When going to bed at the end of work days, pilots subjectively reported sleeping 7.64 ± 1.64 h in the prior 24 h. Significant diurnal and wake-dependent effects were observed for subjective alertness and objective psychomotor performance, with minimum levels occurring between 09:00 and 10:00. Thus, despite their irregular work schedule, ship pilots presented, as a group, a diurnal variation of alertness and psychomotor performance indicative of a day-oriented circadian system. Important inter-individual differences were observed on psychomotor performance mesor and phase. In individuals, earlier phases in psychomotor performance were correlated with earlier chronotype. This study indicates that both circadian and homeostatic processes modulate alertness and psychomotor performance levels with worst levels reached when long shifts ended in the morning. This work has potential applications as it indicates fatigue countermeasures considering both processes are scientifically based.     

Diurnal variation of metabolites in three individual participants

The circadian system influences virtually all biological functions. Understanding the impact of circadian variation on metabolism may provide insight into mechanisms of circadian-associated disorders and guide the implementation of chrono-therapy. Previous research has reported circadian variation in 7–20% of metabolites in human blood. In this study, untargeted metabolomics profiles were measured using blood of two healthy men and one healthy woman, collected every 2 h for up to 48 h under carefully controlled conditions. The pattern of variation of each metabolite over time was examined on each participant separately, using both one- and two-order harmonic models. A total of 100 of 663 metabolites, representing all metabolite categories, showed diurnal rhythmic concentrations that exceeded the Bonferroni threshold (P < 2.5 × 10^?5). Overall, peak times of many metabolites were clustered during the afternoon-midnight, including the majority of amino acids, all peptides, all lysolipids and all phospholipids, whereas the majority of steroids peaked in the morning. We observed substantial inter-individual variation for both peak times and amplitudes in these three subjects. In conclusion, at least 15% of blood metabolites, representing a broad group of biological pathways, exhibit diurnal variation in three participants. The average peak times of most of these metabolites are clustered in morning or afternoon-midnight. Further work is needed to validate and extend this work in more individuals.     

Effects of obstructive sleep apnea on endogenous circadian rhythms assessed during relaxed wakefulness; an exploratory analysis

ABSTRACT

Obstructive sleep apnea (OSA) is associated with hypertension, cardiovascular disease, and a change in the 24 h pattern of adverse cardiovascular events and mortality. Adverse cardiovascular events occur more frequently in the middle of the night in people with OSA, earlier than the morning prevalence of these events in the general population. It is unknown if these changes are associated with a change in the underlying circadian rhythms, independent of behaviors such as sleep, physical activity, and meal intake. In this exploratory analysis, we studied the endogenous circadian rhythms of blood pressure, heart rate, melatonin and cortisol in 11 participants (48 ± 4 years; seven with OSA) throughout a 5 day study that was originally designed to examine circadian characteristics of obstructive apnea events. After a baseline night, participants completed 10 recurring 5 h 20 min behavioral cycles divided evenly into standardized sleep and wake periods. Blood pressure and heart rate were recorded in a relaxed semirecumbent posture 15 minutes after each scheduled wake time. Salivary melatonin and cortisol concentrations were measured at 1–1.5 h intervals during wakefulness. Mixed-model cosinor analyses were performed to determine the rhythmicity of all variables with respect to external time and separately to circadian phases (aligned to the dim light melatonin onset, DLMO). The circadian rhythm of blood pressure peaked much later in OSA compared to control participants (group × circadian phase, p < .05); there was also a trend toward a slightly delayed cortisol rhythm in the OSA group. Rhythms of heart rate and melatonin did not differ between the groups. In this exploratory analysis, OSA appears to be associated with a phase change (relative to DLMO) in the endogenous circadian rhythm of blood pressure during relaxed wakefulness, independent of common daily behaviors.     

Mood and the circadian system: investigation of a circadian component in positive affect

The aim of this study was to test if the pattern of human mood variation across the day is consistent with the hypothesis that self-reports of positive affect (PA) have a circadian component, and self-reports of negative affect (NA) do not. Data were collected under two protocols: normal ambulatory conditions of activity and rest and during a 27 h constant routine (CR) procedure. Mood data were collected every 3 h during the wake span of the ambulatory protocol and hourly during the 27 h CR. In both protocols, rectal temperature data were continuously recorded. In the ambulatory protocol, activity data were also collected to enable estimation of the unmasked (purified) temperature rhythm. Participants were 14 healthy females aged 18-25 yr in the follicular phase of the menstrual cycle. Under both protocols, PA exhibited significant 24 h temporal variation [CR: F(23, 161) = 2.12, p < 0.01; ambulatory: F(5,55) = 2.44, p < 0.05] with a significant sinusoidal component [CR: F(2, 21) = 7.51, p < 0.01; ambulatory: F(2,3) = 20.49, p < .05] of the same form as the circadian temperature rhythm. In contrast, NA exhibited an increasing linear trend over time under the ambulatory protocol [F(1, 11) = 5.74, p < 0.05] but nonsignificant temporal variation under the CR protocol. The findings support the hypothesis of a circadian component in PA variation.     

Post-resistance training detraining: time-of-day effects on training and testing outcomes

The aim of this study was to examine the effects of 3 and 5 weeks of detraining after 14 weeks of resistance training at a specific time of day on performances during the squat jump (SJ) and the maximal voluntary contraction (MVC). Thirty-one healthy male physical education students (age: 23.1 ± 1.0 years; height: 176.1 ± 6.3 cm; weight: 74.9 ± 10.9 kg) were randomly assigned to either a morning training group (MTG, training between 07:00 and 08:00 h, n = 10), an evening training group (ETG, training between 17:00 and 18:00 h, n = 11) or a control group (CG, no training, n = 10). Participants then performed eight test sessions (twice per day, at 07:00 and 17:00 h) over the course of four phases: during pre-training, immediately post-training, and after 3 and 5 weeks of detraining. Before each test session, oral temperature was recorded. During the first 12 weeks of resistance training, participants performed 3 sets of 10 repetitions to failure (10-RM) for 4 exercises (squat, leg press, leg extension and leg curl, with 2 min of recovery between each exercise); during the last two weeks, training intensity increased to 8-RM with 3 min of recovery between each exercise. Oral temperature was significantly higher at 17:00 than 07:00 h during all test periods (p < 0.05). Likewise, SJ and MVC performances were significantly higher at 17:00 h than 07:00 h during all four test days in ETG and CG, and before training and 3 and 5 weeks after training in MTG (p < 0.05). For both training groups, most SJ and MVC performances (except MTG at 07:00 h and ETG at 17:00 h) returned to baseline values after 5, but not after 3, weeks of detraining. This study showed that 14 weeks of training at a specific time of day blunted the diurnal variation of MVC and SJ in the MTG. The improvement in performance brought about by resistance training was partially retained after 3 weeks of detraining (unless training had taken place at a non-habitual time of day) but was lost after 5 weeks of detraining. There was no effect of the time of training on core temperature.     

Daily rhythms in activity and mRNA abundance of enzymes involved in glucose and lipid metabolism in liver of rainbow trout,Oncorhynchus mykiss. Influence of light and food availability

ABSTRACT

We have investigated the magnitude of diurnal variation in back squat and bench press performance using the MuscleLab force velocity transducer. Thirty resistance-trained males (mean ± SD: age 21.7 ± 1.4 years; body mass 80.5 ± 4.5 kg; height 1.79 ± 0.06 m) underwent two sessions at different times of day: morning (M, 07:30 h) and evening (E, 17:30 h). Each session included a period when rectal temperature (T_rec) was measured at rest, a 5-min standardized 150 W warm-up on a cycle ergometer, then defined programme of bench press (at 20, 40 and 60 kg) and back squat (at 30, 50 and 70 kg) exercises. A linear encoder was attached to an Olympic bar used for the exercises and average force (AF), peak velocity (PV) and time-to-peak velocity (tPV) were measured (MuscleLab software; MuscleLab Technology, Langesund, Norway) during the concentric phase of the movements. Values for T_rec at rest were higher in the evening compared to morning values (0.48°C, P < 0.0005). Daily variations were apparent for both bench press and back squat performance for AF (1.9 and 2.5%), PV (8.3 and 12.7%) and tPV (?16.6 and ?9.8%; where a negative number indicates a decrease in the variable from morning to evening). There was a main effect for load where AF and tPV increased and PV decreased from the lightest load to the heaviest for both bench press and back squat (47.1 and 80.2%; 31.7 and 57.7%; ?42.1 and ?73.9%; P < 0.0005 where a negative number indicates a decrease in the variable with increasing load). An interaction was found only for tPV, such that the tPV occurs earlier in the evening than the morning at the highest loads (60 and 70 kg) for both bench press and back squat, respectively (mean difference of 0.32 and 0.62 s). In summary, diurnal variation in back squat and bench press was shown; and the tPV in complex multi-joint movements occurs earlier during the concentric phase of exercise when back squat or bench press is performed in the evening compared to the morning. This difference can be detected using a low cost, portable and widely available commercial instrument and enables translation of past laboratory/tightly controlled experimental research in to main-stream coaching practice.     

Comparing the response to acute and chronic exposure to short wavelength lighting emitted from computer screens

The use of electronic devices with light-emitting screens has increased exponentially in the last decade. As a result, humans are continuously exposed to unintentional artificial light. We explored the effects of acute and chronic exposure to artificial light at night (ALAN) via screen illumination on sleep, circadian rhythms, and related functional outcomes. Nineteen participants (11 female and 8 males, mean age 28.1 ± 7.2 years) underwent a six-night study with three experimental conditions using a repeated-measures design: baseline (first night, no light exposure), acute ALAN exposure (second night), and chronic ALAN exposure (third to sixth nights). Each light exposure lasted for 2 hours (21:00–23:00). Participants underwent an overnight polysomnography at the end of each condition (nights 1, 2, and 6). We collected urine samples (for melatonin metabolite analysis), while body (oral) temperatures were measured before and after exposure. Each morning, the participants filled out questionnaires and conducted a computerized attention test. Both acute and chronic illumination significantly disrupted sleep continuity and architecture and led to greater self-reported daytime sleepiness, negative emotions, and attention difficulties. Both exposure types also altered circadian rhythms, subduing the normal nocturnal decline in body temperature and dampening nocturnal melatonin secretion. In sum, ALAN exposure from electronic screens has an immediate, detrimental, yet stable effect on sleep, circadian regulation, and next-day functional outcomes. Given the widespread use of electronic devices today, our findings suggest that even one night of screen light exposure may be sufficient to cause adverse effects on health and performance.     

Diurnal variations of postural stability and attentional capacities in 5–6-year-old tennis players

The present study aimed to investigate the diurnal variation of postural stability, attentional capacities, and oral temperature in 5–6-year-old tennis players. A total of 24 young children were divided into two groups: Twelve tennis players with 2 years of training experience and twelve sedentary children without previous experience in any type of sport. They were asked to maintain an upright bipedal stance on a static force platform with eyes open (EO) and eyes closed (EC) at 07:00, 10:00, 14:00, and 18:00 h. Postural stability was evaluated by the center of pressure (CoP), surface area (CoP_Area), CoP mean velocity (CoP_Vm), and Romberg’s index (RI). Oral temperature and simple reaction time (SRT) were also recorded at the beginning of each test session. Postural stability in 5–6-year-old tennis players was low at 07:00 h and improved at 10:00 h (p < 0.001); then, it worsened at 14:00 h (p < 0.001) and improved again at 18:00 h (p < 0.001) as found in sedentary children. This rhythm was also close to that of SRT in both groups. Body temperature increased significantly (p < 0.001) from the morning to the afternoon in both groups. However, the peak of postural performance, the peak of attentional capacities, and the greatest vision contribution to maintain balance observed at 18:00 h were only found in the trained group. Moreover, young tennis players were more stable (p < 0.001) and more attentive (p < 0.01) than sedentary ones at 18:00 h. The amplitude of the diurnal rhythm of CoP parameters was significantly larger (p < 0.01) in trained children compared to sedentary ones (43.1 ± 8.5 vs 31.7 ± 8.3 for the CoP_Area; 27.5 ± 7.4 vs 17.7 ± 8.2 for the CoP_Vm). Therefore, time-of-day has a significant effect on static postural stability and attentional capacities in young tennis players with better performances in the late afternoon (habitual hour of training). Thus, we recommend planning some training sessions at 07:00 and/or 14:00 h to optimize postural stability at the time of its batyphase and to reduce the incidence of fall-related injury risks during this critical age in athletic environments.     

Pronounced juvenile circadian core temperature rhythms exist in several strains of rats but not in rabbits

Torpor-like circadian variations of core temperature are well documented for suckling-age Zucker rat pups. To determine (1) whether this juvenile circadian rhythm is as strongly expressed in other rat strains, and (2) whether a similar rhythm is expressed in rabbit pups, we recorded core temperature and metabolic rate of artificially reared pups. Wistar, Brown Norway, and Long Evans pups were studied for 30 h under moderate cold loads (ambient temperature=28°C) when 9–11 days old, i.e. at the age and ambient temperature for which the rhythm has been most thoroughly characterized in Zucker rats. Chinchilla bastard rabbit pups were studied under similar conditions when they were 3–8 days old, the youngest age at which the rhythm can be easily detected in rats. Rat pups of each strain showed clear circadian rhythms with sharp decreases of core temperature and metabolic rate in subjective morning. Core temperature amplitudes were in the order Wistar < Brown Norway < Zucker < Long Evans strain. In contrast, the rabbit pups maintained stable high levels of core temperature and metabolic rate throughout the day. A torpor-like decrease of core temperature in the morning is thus not a pecularity of the Zucker rat strain but also occurs in other pigmented rat strains, whereas rabbit pups at a similar developmental stage do not show a circadian core temperature rhythm.