Circadian aspects of body temperature regulation in exercise

Internal and external factors contribute to resting core temperature and affect thermoregulation. Also, a robust circadian rhythm exists, implying that the body is in “heat-gain” or “heat-loss” modes at different times during the 24 h. Moreover, many variables associated with exercise, and the body's capacity for exercise, show circadian variation. All these factors contribute to circadian changes in thermoregulation during exercise. Attention is focused on responses at the onset of exercise, “critical temperature”, and recovery after exercise. Practical implications of circadian changes in thermoregulation during exercise include ergogenic aids and inter-individual differences, including those due to gender, age and acclimatisation.     

Prolonged mild hypoxia modifies human circadian core body temperature and may be associated with sleep disturbances

Fatigue is often reported after long-haul airplane flights. Hypobaric hypoxia, observed in pressurized cabins, may play a role in this phenomenon by altering circadian rhythms. In a controlled cross-over study, we assessed the effects of two levels of hypoxia, corresponding to cabin altitudes of 8000 and 12,000 ft, on the rhythm of core body temperature (CBT), a marker of circadian rhythmicity, and on subjective sleep. Twenty healthy young male volunteers were exposed for 8 h (08:00-16:00 h) in a hypobaric chamber to a cabin altitude of 8000 ft and, 4 weeks later, 12,000 ft. Each subject served as his own control. For each exposure, CBT was recorded by telemetry for two 24 h cycles (control and hypoxic exposure). After filtering out nonphysiological values, the individual CBT data were fitted with a five-order moving average before statistical group analysis. Sleep latency, sleep time, and sleep efficiency were studied by sleep logs completed every day in the morning. Our results show that the CBT rhythm expression was altered, mainly at 12,000 ft, with a significant increase of amplitude and a delay in the evening decline in CBT, associated with alterations of sleep latency. Mild hypoxia may therefore alter circadian structure and result in sleep disturbances. These results may explain in part the frequent complaints of prolonged post-flight fatigue after long flights, even when no time zones are crossed.     

Core temperature response to cycling exercise: Effect of time of day and measurement site

This study evaluated the effect of time of day and temperature measurement site on core temperature response to exercise. Six trained cyclists performed a 1 h cycling exercise at a fixed power-output of 160 W in a controlled environment (ambient temperature of 21.5±1.6 °C and relative humidity of 31±6%) at batyphase +2 h (08:00 h) and acrophase +2 h (20:00 h) of their estimated circadian temperature rhythm; corresponding respectively to the heat gain and heat loss mode phases. Throughout the exercise, rectal and gastro-intestinal temperature data were collected. A two-way ANOVA was applied and a common nonlinear logistic-type function dependant on three parameters (asymptote, xmid and scale) was used to fit collected data. ANOVA only indicated a time of day effect without interaction with exercise duration. A nonlinear mixed-effect model allowed further analysis of temperature kinetics. The model indicated a higher theoretical increase in temperature at the end of morning exercise compared to the evening session. However, the circadian difference observed at rest persists throughout the exercise. Theoretical asymptotic temperature values at the end of the exercise and scale values (inversely proportional to the slope) are higher for the rectal measurement site than for the gastro-intestinal measurement. The model proposed offers a solution for refining the study of individual core temperature response to prolonged exercise. The main advantage is that it takes into consideration intra- and inter-individual variability in temperature kinetics.     

Pharmacological effects of vinorelbine on body temperature and locomotor activity circadian rhythms in mice

The effects of vinorelbine (VRL) on the circadian rhythms in body temperature and locomotor activity were investigated in unrestrained B6D2F₁ mice implanted with radio-telemetry transmitters. A single intravenous VRL dose (24 or 12 mg/kg) was given at 7 h after light onset (HALO), a time of high VRL toxicity, and resulted in transient suppression of temperature and activity circadian rhythms in mice kept in light-dark (LD) 12h:12h. Such suppression was dose-dependent. It occurred within 1-5 d after VRL dosing. Recovery of both rhythms was partially complete within 5 d following the high dose and within 2 or 3 d after the low dose and was not influenced by suppression of photoperiodic synchronization by housing in continuous darkness. Moreover, VRL induced a dose-dependent relative decrease in amplitude and phase shift of the temperature circadian rhythm. The mesor and amplitude of the activity rhythm were markedly reduced following the VRL administration. The relevance of VRL dosing time was studied in mice housed in LD 12h:12h. Vinorelbine was injected weekly (20 mg/kg/injection) for 3 wk at 6 or 18 HALO. Vinorelbine treatment ablated the rest-activity and temperature rhythms 3-6 d after each dose, with fewer alterations after VRL dosing at 18 HALO compared to 6 HALO, especially for the body temperature rhythm. There was at least partial recovery 1 wk after dosing, suggesting the weekly schedule of drug treatment is acceptable for therapeutic purposes. Our findings demonstrate that VRL can transiently, yet profoundly, alter circadian clock function. Vinorelbine-induced circadian dysfunction may contribute to the toxicokinetics of this and possibly other anticancer drugs.     

Disturbance of circadian rhythms in analgosedated intensive care unit patients with and without craniocerebral injury

Melatonin, cortisol, heart rate, blood pressure, spontaneous motor activity, and body temperature follow stable circadian rhythms in healthy individuals. These circadian rhythms may be influenced or impaired by the loss of external zeitgebers during analgosedation, critical illness, continuous therapeutic intervention in the intensive care unit (ICU), and cerebral injury. This prospective, observational, clinical study examined 24 critically ill analgo-sedated patients, 13 patients following surgery, trauma, or acute respiratory distress (ICU), and 11 patients with acute severe brain injury following trauma or cerebral hemorrhage (CCI). Blood samples for the determination of melatonin and cortisol were obtained from each patient at 2 h intervals for 24 h beginning at 18:00 h on day 1 and ending 16:00 h on day 2. Blood pressure, heart rate, body temperature, and spontaneous motor activity were monitored continuously. Level of sedation was assessed using the Ramsey Sedation Scale. The severity of illness was assessed using the APACHE-II-score. The time series data were analyzed by rhythm analysis with the Chronos-Fit program, using partial Fourier series with up to six harmonics. The 24 h profiles of all parameters from both groups of patients were greatly disturbed/abolished compared to the well-known rhythmic 24 h patterns in healthy controls. These rhythm disturbances were more pronounced in patients with brain injury. The results of this study provide evidence for a pronounced disturbance of the physiological temporal organization in ICU patients. The relative contribution of analgosedation and/or brain injury, however, is a point of future investigation.     

The circadian rhythm of core temperature: origin and some implications for exercise performance

This review first examines reliable and convenient ways of measuring core temperature for studying the circadian rhythm, concluding that measurements of rectal and gut temperature fulfil these requirements, but that insulated axilla temperature does not. The origin of the circadian rhythm of core temperature is mainly due to circadian changes in the rate of loss of heat through the extremities, mediated by vasodilatation of the cutaneous vasculature. Difficulties arise when the rhythm of core temperature is used as a marker of the body clock, since it is also affected by the sleep-wake cycle. This masking effect can be overcome directly by constant routines and indirectly by “purification” methods, several of which are described. Evidence supports the value of purification methods to act as a substitute when constant routines cannot be performed. Since many of the mechanisms that rise to the circadian rhythm of core temperature are the same as those that occur during thermoregulation in exercise, there is an interaction between the two. This interaction is manifest in the initial response to spontaneous activity and to mild exercise, body temperature rising more quickly and thermoregulatory reflexes being recruited less quickly around the trough and rising phase of the resting temperature rhythm, in comparison with the peak and falling phase. There are also implications for athletes, who need to exercise maximally and with minimal risk of muscle injury or heat exhaustion in a variety of ambient temperatures and at different times of the day. Understanding the circadian rhythm of core temperature may reduce potential hazards due to the time of day when exercise is performed.     

Circadian variation in oxidative stress markers in healthy and type II diabetic men

Seven clinically healthy, nondiabetic (ND) and four Type II diabetic (D) men were assessed for circadian rhythms in oxidative “stress markers.” Blood samples were collected at 3h intervals for ∼27 h beginning at 19:00h. Urine samples were collected every 3 h beginning with the 16:00h-19:00h sample. The dark (sleep) phase of the light-dark cycle extended from 22:30h to 06:30h, with brief awakening for sampling at 01:00h and 04:00h. Subjects were offered general hospital meals at 16:30h, 07:30h, and 13:30h (2400 cal in total/24 h). Serum samples were analyzed for uric acid (UA) and nitrite (NO) concentrations, and urine samples were assayed for 8-hydroxydeoxyguanosine (8-OHdG), malondialdehyde (MDA), and 8-isoprostane (ISP). Data were analyzed statistically both by the population multiple-components method and by the analysis of variance (ANOVA). The 24h mean level of UA and NO was greater in D than in ND subjects (424 vs. 338 μmol/L and 39.2 vs. 12.7 μM, respectively). A significant circadian rhythm in UA (p=0.001) and NO (p=0.048) was evident in ND but not in D (p=0.214 and 0.065). A circadian rhythm (p=0.004, amplitude=8.6 pmol/kgbw/3h urine vol.) was also evident in urine 8-OHdG of ND but not of D. The 24h mean levels of ND and D were comparable (76.8 vs. 65.7 pmol/kgbw/3h urine vol.). No circadian rhythm by population multiple-components was evident in MDA and ISP levels of ND subjects, or in 8-OHdG, MDA, and ISP in D. However, a significant time-effect was demonstrated by ANOVA in all variables and groups. The 24h mean of MDA and ISP in D was significantly greater than in ND (214 vs. 119 nmol/3h urine vol. and 622 vs. 465 ng/3h urine vol.). The peak concentrations of the three oxidative “stress markers” in urine, like those of serum NO, occurred early in the evening in both groups of men. This observation suggests a correlation between increased oxidative damage and increased rate of anabolic-catabolic events as evidenced by similarities in the timing of peak NO production and in parameters relevant to metabolic functions.     

Reproductive phase dependent circadian variations of plasma melatonin, testosterone, thyroxine and corticosterone in Indian palm squirrel, Funambulus pennanti

To explain the complex mechanism of environmental influence along with internal hormonal (factors) milieu on daily variations in the circulating levels of melatonin, testosterone, thyroxine and corticosterone were analyzed with the help of inferential statistics (Cosinor rhythmometry) in a seasonally breeding tropical rodent, F. pennanti during the reproductively active (RAP) and inactive phases (RIP). Plasma melatonin, thyroxine and corticosterone levels exhibited a significant circadian oscillation during both the active and inactive phases of the annual reproductive cycle. Melatonin showed higher amplitude during RIP in the circulating plasma. Testosterone presented a peak level during evening hours (16:00 - 18:00 h) during RAP only. The phase of thyroxine was noted ∼09:76 h and ∼10.35 h during active and inactive phases, respectively. Corticosterone showed a peak level at ∼12.00 h during both phases of the reproductive cycle. Further, in this tropical rodent, the minimum difference in photoperiod (∼3 - 4 hours) and maximum variation in temperature (max. 18°C - min. 10°C during RIP and max. 45°C - min. 32°C during RAP) and humidity (85% during RIP and 35% during RAP) regulated the diurnal rhythm of circulating melatonin circadian rhythm by ∼1 hour phase advance during RIP. In conclusion, the studied hormonal rhythms may be part of an integrative system to coordinate reproduction and physiological processes successfully with environmental factors.     

中缅树鼩体温、代谢率和蒸发失水日节律

为研究中缅树鼩(Tupaia belangeri)体温、代谢率和蒸发失水的日节律变化,采用植入式体温计测定了中缅树鼩24 h的体温,以及24 h中4个时间段(05:00～07:00时、11:00～13:00时、17:00～19:00时和23:00～01:00时)热中性区(30℃)的静止代谢率(RMR)、非颤抖性产热(NST)和蒸发失水(EWL)。结果显示,中缅树鼩的体温具有日节律变化,最高值和最低值分别出现在11:00时和03:00时,各为(39.45±0.26)℃和(36.34±0.24)℃;静止代谢率、非颤抖性产热和蒸发失水在4个时间段都有显著差异,表现出显著的日节律变化,代谢率在23:00～01:00时最大,O2含量为(2.58±0.04)ml/(g.h),在11:00～13:00时最小,O2含量为(2.28±0.09)ml/(g.h);非颤抖性产热在05:00～07:00时最大,O2含量为(3.08±0.14)ml/(g.h),在11:00～13:00最小,O2含量为(2.69±0.63)ml/(g.h);蒸发失水在17:00～19:00时最大,失水量为(3.60±0.31)mg/(g.h)。结果表明,体温的日节律变化主要与环境温度的日节律变化和下午出窝取食活动性增强有关;当夜晚环境温度相对较低的时候,通过增强静止代谢率和非颤抖性产热来增加产热,而白天环境温度相对较高的时候,通过增强蒸发失水散热来调节体温。     

The Circadian Rhythm of Core Temperature: Origin and some Implications for Exercise Performance

This review first examines reliable and convenient ways of measuring core temperature for studying the circadian rhythm, concluding that measurements of rectal and gut temperature fulfil these requirements, but that insulated axilla temperature does not. The origin of the circadian rhythm of core temperature is mainly due to circadian changes in the rate of loss of heat through the extremities, mediated by vasodilatation of the cutaneous vasculature. Difficulties arise when the rhythm of core temperature is used as a marker of the body clock, since it is also affected by the sleep‐wake cycle. This masking effect can be overcome directly by constant routines and indirectly by “purification” methods, several of which are described. Evidence supports the value of purification methods to act as a substitute when constant routines cannot be performed. Since many of the mechanisms that rise to the circadian rhythm of core temperature are the same as those that occur during thermoregulation in exercise, there is an interaction between the two. This interaction is manifest in the initial response to spontaneous activity and to mild exercise, body temperature rising more quickly and thermoregulatory reflexes being recruited less quickly around the trough and rising phase of the resting temperature rhythm, in comparison with the peak and falling phase. There are also implications for athletes, who need to exercise maximally and with minimal risk of muscle injury or heat exhaustion in a variety of ambient temperatures and at different times of the day. Understanding the circadian rhythm of core temperature may reduce potential hazards due to the time of day when exercise is performed.     

Circadian variation in cardiac autonomic activity: reactivity measurements to different types of stressors

The role of endogenous circadian rhythmicity in autonomic cardiac reactivity to different stressors was investigated. A constant routine protocol was used with repeated exposure to a dual task and a cold pressor test. The 29 subjects were randomly divided into two groups in order to manipulate prior wakefulness. Group 1 started at 09:00 h immediately after a monitored sleep period, whereas group 2 started 12 h later. Measures of interbeat intervals (IBI), respiratory sinus arrythmia (RSA, a measure of parasympathetic activity), pre-ejection period (PEP, a measure of sympathetic activity), as well as core body temperature (CBT) were recorded continuously. Multilevel regression analyses (across-subjects) revealed significant (mainly 24 h) sinusoidal circadian variation in the response to both stressors for IBI and RSA, but not for PEP. Individual 24 + 12 h cosine fits demonstrated a relatively large interindividual variation of the phases of the IBI and RSA rhythms, as compared to that of the CBT rhythm. Sinusoidal by group interactions were found for IBI and PEP, but not for RSA. These findings were interpreted as an indication for endogenous circadian and exogenous parasympathetic (vagal) modulation of cardiac reactivity, while sympathetic reactivity is relatively unaffected by the endogenous circadian drive and mainly influenced by exogenous factors.     

Diazepam affects both level and amplitude of rat locomotor activity rhythm but has no effect on core body temperature

The present study investigates the effects of a chronic administration of diazepam, a benzodiazepine widely used as an anxiolytic, on locomotor activity and body core temperature rhythms in male Wistar rats housed under 12:12 light:dark (LD) cycle conditions. Diazepam was administered subcutaneously for 3 wks in a dosage of 3 mg/kg body weight/day, 1 h before the onset of darkness. Diazepam increased the level of locomotor activity from the first day until the end of treatment, and also increased the amplitude of the activity circadian rhythm, but only on the third wk of treatment. Diazepam exerted no effects on the length of the period and did not affect the phase of the locomotor activity rhythm. The body temperature rhythm of rats was affected neither by short-term (a single injection) nor by long-term (every day for 3 wks) diazepam treatment. Diazepam lacked effect on body core temperature even on the first day of administration, thereby ruling out the possibility of drug tolerance development. The fact that diazepam affects locomotor activity, but not core body temperature, suggests that different mechanisms mediate the actions of diazepam on locomotor activity and on core body temperature.     

同步遥测分析大鼠棕色脂肪产热与体温昼夜节律变化的时间关系

目的：同步遥测棕色脂肪组织（BAT）产热与体核温度昼夜律变化的时间曲线,分析二者昼夜节律变化的时间关系。方法：实验用成年雄性SD大鼠,在22℃环境温度下,明暗时间各12h,昼光时间为06：00h-18：00h,同步无线遥测体核温度（TC）、BAT温度（T队T）、腋窝温度（Tax）和动物活动的昼夜节律变化。结果：①在昼光中,TBAT较TC低0．67％,而在暗光中二者则相似。大鼠从昼光进入暗光时,TBAT升高的速率较TC升高速率快,开始上升的时间较TC提前8min;而从暗光进入昼光时,TBAT开始下降的时间则较TC提前4min。②Tax的昼夜节律幅度与TC相似,但无论动物在明光期或暗光期中,Tax均低于同步测量的TC。③从昼光期转入暗光期时,动物的行为活动出现增加反应先于TBAT和TC开始上升的时间。结论：实验结果证明,在暗光期中大鼠TC升高与BAT产热增加有关,说明BAT昼夜节律性产热的变化在维持体温昼夜生理节律中有重要的作用。     

Effects of a five-hour exposure of arm and leg to a slightly cool thermal environment in the afternoon on the body temperature rhythm

The purpose of this study was to determine whether the different thermal conditions of the arm and leg due to wearing different types of clothing during the afternoon could modulate the circadian rhythm of body temperature and subjective sleep quality. Six healthy female volunteers were studied twice with two types of clothing, leaving the arm and leg covered or uncovered. The environmental chamber was controlled at 24 ± 0.5°C and 50 ± 5% RH during wakefulness and 28 ± 0.5°C and 50 ± 5% RH during night sleep. One type of clothing consisted of long-sleeved shirts and full-length trousers (Type L, 989 g, 0.991 clo); the other type was of half-sleeved shirts and knee-length trousers (Type H, 750 g, 0.747 clo). One testing session lasted for 16 h from 14:00 to 06:00 h the next morning. Subjects wore Type L or Type H clothing during the afternoon exposure (14:00 - 19:00), and Type L clothing during the evening (19:00 - 22:30 h) and the night sleep (22:30 - 06:00 h). Results were as follows. (1) When wearing Type H rather than Type L clothing, skin temperatures of the arm and leg were significantly lower during the time of exposure, and increased more after the evening. (2) Rectal temperature was not significantly different between the two types of clothing except during the early part of the exposure period, but it decreased during the evening by a significantly greater amount when wearing Type H clothing. (3) Subjective sleep quality was not significantly different between the two clothing types. These results suggest that afternoon exposure of the arm and leg to a slightly cool environment does not have a strong after-effect on the body temperature and subjective sleep quality.     

Ring the Bell for Matins: Circadian Adaptation to Split Sleep by Cloistered Monks and Nuns

Cloistered monks and nuns adhere to a 10-century-old strict schedule with a common zeitgeber of a night split by a 2- to 3-h-long Office (Matins). The authors evaluated how the circadian core body temperature rhythm and sleep adapt in cloistered monks and nuns in two monasteries. Five monks and five nuns following the split-sleep night schedule for 5 to 46 yrs without interruption and 10 controls underwent interviews, sleep scales, and physical examination and produced a week-long sleep diary and actigraphy, plus 48-h recordings of core body temperature. The circadian rhythm of temperature was described by partial Fourier time-series analysis (with 12- and 24-h harmonics). The temperature peak and trough values and clock times did not differ between groups. However, the temperature rhythm was biphasic in monks and nuns, with an early decrease at 19:39?±?4:30?h (median?±?95% interval), plateau or rise of temperature at 22:35?±?00:23?h (while asleep) lasting 296?±?39?min, followed by a second decrease after the Matins Office, and a classical morning rise. Although they required alarm clocks to wake-up for Matins at midnight, the body temperature rise anticipated the nocturnal awakening by 85?±?15?min. Compared to the controls, the monks and nuns had an earlier sleep onset (20:05?±?00:59?h vs. 00:00?±?00:54?h, median?±?95% confidence interval, p?=?.0001) and offset (06:27?±?0:22?h, vs. 07:37?±?0:33?h, p?=?.0001), as well as a shorter sleep time (6.5?±?0.6 vs. 7.6?±?0.7?h, p?=?.05). They reported difficulties with sleep latency, sleep duration, and daytime function, and more frequent hypnagogic hallucinations. In contrast to their daytime silence, they experienced conversations (and occasionally prayers) in dreams. The biphasic temperature profile in monks and nuns suggests the human clock adapts to and even anticipates nocturnal awakenings. It resembles the biphasic sleep and rhythm of healthy volunteers transferred to a short (10-h) photoperiod and provides a living glance into the sleep pattern of medieval time. (Author correspondence: isabelle.arnulf@psl.aphp.fr)     

Sleepiness and sleep in a simulated "six hours on/six hours off" sea watch system

Ships are operated around the clock using rapidly rotating shift schedules called sea watch systems. Sea watch systems may cause fatigue, in the same way as other irregular working time arrangements. The present study investigated subjective sleepiness and sleep duration in connection with a 6 h on/6 h off duty system. The study was performed in a bridge simulator, very similar to those found on ships. Twelve officers divided into two groups participated in the study that lasted 66 h. Half of the subjects started with the 06:00-12:00 h watch and the other half with the 12:00-18:00 h watch. The subjects alternated between off-duty and on-duty for the remainder of the experimental period. Approximately halfway through the experiment, the 12:00-18:00 h watch was divided into two 3 h watches/off-duty periods. The effect of this was to reverse the on-duty/off-duty pattern between the two groups. This enabled all subjects to work the four possible watches (00:00-06:00 h, 06:00-12:00 h, 12:00-18:00 h, and 18:00-24:00 h) in an order that was essentially counterbalanced between groups. Ratings of sleepiness (Karolinska Sleepiness Scale; KSS) were obtained every 30 min during on-duty periods and if subjects were awake during off-duty periods. The subjectively rated duration of sleep was recorded after each off-duty period that preceded watch periods when KSS was rated. The results showed that the average level of sleepiness was significantly higher during the 00:00-06:00 h watch compared to the 12:00-18:00 h and 18:00-24:00 h watches, but not to the 06:00-12:00 h watch. Sleepiness also progressed significantly from the start toward the end of each watch, with the exception of the 06:00-12:00 h watch, when levels remained approximately stable. There were no differences between groups (i.e., the order between watches). Sleep duration during the 06:00-12:00 h off-duty period (3 h 29 min) was significantly longer than during the 12:00-18:00 h period (1 h 47 min) and the 18:00-24:00 h period (2 h 7 min). Sleep during the 00:00-06:00 h period (4 h 23 min) was longer than all sleep periods except the 06:00-12:00 h period. There were no differences between groups. In spite of sufficient opportunities for sleep, sleep was on the average around 1-1 h 30 min shorter than the 7-7 h 30 min that is considered “normal” during a 24 h period. This is probably a consequence of the difficulty to sleep during daytime due to the alerting effects of the circadian rhythm. Also, sleepiness during the night and early mornings reached high levels, which may be explained by a combination of working close to or during the circadian trough of alertness and the relatively short sleep periods obtained. An initial suppression of sleepiness was observed during all watches, except for the 06:00-12:00 h watch. This suppression may be explained by the “masking effect” exerted by the relative high levels of activity required when taking over the responsibility of the ship. Toward the end of watches, the levels of sleepiness progressively increased to relatively high levels, at least during the 00:00-06:00 h watch. Presumably, initially high levels of activity are replaced by routine and even boredom.     

Circadian response of annual weeds to glyphosate and glufosinate

Five field experiments were conducted in 1998 and 1999 in Minnesota to examine the influence of time of day efficacy of glyphosate [N-(phosphonomethyl)glycine] and glufosinate [2-amino-4-(hydroxymethyl-phosphinyl)butanoic acid] applications on the control of annual weeds. Each experiment was designed to be a randomized complete block with four replications using plot sizes of 3×9 m. Glyphosate and glufosinate were applied at rates of 0.421 kg ae/ha and 0.292 kg ai/ha, respectively, with and without an additional adjuvant that consisted of 20% nonionic surfactant and 80% ammonium sulfate. All treatments were applied with water at 94 L/ha. Times of day for the application of herbicide were 06:00h, 09:00h, 12:00h, 15:00h, 18:00h, 21:00h, and 24:00h. Efficacy was evaluated 14 d after application by visual ratings. At 14 d, a circadian response to each herbicide was found, with greatest annual weed control observed with an application occurring between 09:00h and 18:00h and significantly less weed control observed with an application at 06:00h, 21:00h, or 24:00h. The addition of an adjuvant to both herbicides increased overall efficacy, but did not overcome the rhythmic time of day effect. Results of the multiple regression analysis showed that after environmental temperature, time of day was the second most important predictor of percent weed kill. Thus, circadian timing of herbicide application significantly influenced weed control with both glyphosate and glufosinate.     

A comparison of the immediate effects of moderate exercise in the late morning and late afternoon on core temperature and cutaneous thermoregulatory mechanisms

Twelve healthy male subjects each undertook two bouts of moderate exercise (70% VO2max for 30 minutes) in the morning (08:00) and late afternoon (18:00) at least 4 days apart. Measurements were made of heart rate, core (rectal) temperature, sternum skin temperature, and forearm skin blood flow during baseline conditions, during the bout of exercise, and throughout a 30-minute recovery period. Comparisons were made of the changes of heart rate, temperature, and skin blood flow produced by the exercise at the two times of day. Student t tests indicated that baseline values for core temperature (37.15 degrees C +/- 0.06 degrees C vs. 36.77 degrees C +/- 0.06 degrees C) and sternum temperature (33.60 degrees C +/- 0.29 degrees C vs. 32.70 degrees C + 0.38 degrees C) were significantly (p < .05) higher in the late afternoon than the early morning. Two-way analysis of variance (ANOVA) indicated that the increases in core and sternum temperatures during exercise were significantly less (p = .0039 and .0421, respectively) during the afternoon bout of exercise compared with the morning, even though the work loads, as determined by changes in heart rate, were not significantly different (p = .798) at the two times of testing. There were also tendencies for resting forearm skin blood flow to be higher in the afternoon than in the morning and for exercise to produce a more rapid rise in this variable in the afternoon. The possible mechanisms producing these responses to exercise are discussed in terms of those that are responsible for the normal circadian rhythm of core temperature. It is concluded that the body's ability to remove a heat load is less in the early morning, when the circadian system is in a "heat gain" mode, than in the late afternoon, when heat gain and "heat loss" modes are balanced more evenly.     

Ring the bell for Matins: circadian adaptation to split sleep by cloistered monks and nuns

Cloistered monks and nuns adhere to a 10-century-old strict schedule with a common zeitgeber of a night split by a 2- to 3-h-long Office (Matins). The authors evaluated how the circadian core body temperature rhythm and sleep adapt in cloistered monks and nuns in two monasteries. Five monks and five nuns following the split-sleep night schedule for 5 to 46 yrs without interruption and 10 controls underwent interviews, sleep scales, and physical examination and produced a week-long sleep diary and actigraphy, plus 48-h recordings of core body temperature. The circadian rhythm of temperature was described by partial Fourier time-series analysis (with 12- and 24-h harmonics). The temperature peak and trough values and clock times did not differ between groups. However, the temperature rhythm was biphasic in monks and nuns, with an early decrease at 19:39 ± 4:30 h (median ± 95% interval), plateau or rise of temperature at 22:35 ± 00:23 h (while asleep) lasting 296 ± 39 min, followed by a second decrease after the Matins Office, and a classical morning rise. Although they required alarm clocks to wake-up for Matins at midnight, the body temperature rise anticipated the nocturnal awakening by 85 ± 15 min. Compared to the controls, the monks and nuns had an earlier sleep onset (20:05 ± 00:59 h vs. 00:00 ± 00:54 h, median ± 95% confidence interval, p= .0001) and offset (06:27 ± 0:22 h, vs. 07:37 ± 0:33 h, p= .0001), as well as a shorter sleep time (6.5 ± 0.6 vs. 7.6 ± 0.7 h, p= .05). They reported difficulties with sleep latency, sleep duration, and daytime function, and more frequent hypnagogic hallucinations. In contrast to their daytime silence, they experienced conversations (and occasionally prayers) in dreams. The biphasic temperature profile in monks and nuns suggests the human clock adapts to and even anticipates nocturnal awakenings. It resembles the biphasic sleep and rhythm of healthy volunteers transferred to a short (10-h) photoperiod and provides a living glance into the sleep pattern of medieval time.     

Blunting of circadian rhythms and increased acrophase variability in sleep-time hypertensive subjects

24 h and ultradian rhythms of blood pressure (BP) have been previously shown to be disorganized in nocturnal hypertensive subjects. The present study was undertaken to further analyze the ultradian and circadian BP rhythm structure in sleep-time hypertensive subjects with normal or elevated awake-time BP levels. Fourier analysis was used to fit 24, 12, 8, and 6 h curves to mean BP as well as heart rate (HR) time series data derived from 24 h ambulatory blood pressure monitoring. Awake and sleep periods were defined according to individual sleep diaries. Awake-time hypertension was defined as diurnal systolic (SBP) and/or diastolic BP (DBP) means ≥135/85 mmHg. Sleep-time hypertension was defined as nocturnal SBP and/or DBP means ≥120/70 mmHg. The sample included 240 awake-time normotensive subjects (180 sleep-time normotensives and 60 sleep-time hypertensives) and 138 untreated awake-time hypertensive subjects (31 sleep-time normotensives and 107 sleep-time hypertensives). The amplitude and integrity (i.e., percent rhythm) of the 24 and 12 h BP rhythms were lower in the sleep-time hypertensive subjects and higher in the awake-time hypertensive subjects. However, no differences were detected when the integrity and amplitude of the 6 and 8 h mean BP rhythms were analyzed. The sleep-time hypertensive group showed significantly higher 24 h BP rhythm acrophase variability. No differences could be found in any of the HR rhythm parameters. Altogether, the findings suggest a disorganization of the BP circadian rhythm in sleep-time hypertensives that results in reduced 24 h rhythm amplitude and integrity that could be related to cardiovascular risk.