Circadian Rhythms in Competitive Sabre Fencers: Internal Desynchronization and Performance

During a 7-10 day span, circadian rhythms of sleep-wake, self-rated fatigue and mood, oral temperature, eye-hand skill and right and left hand grip strength were investigated in eight subjects: five males (21-28 years of age), members of the French sabre fencing team selected for the 1984 Olympic Games in Los Angeles, and three females (19-26 years of age) practicing fleuret (foil) fencing as a sports activity. On the average six measurements/day/variable/subject were performed. The single cosinor method showed that a circadian rhythm was detectable for only 26 out of the 56 time series (46.4%). Power spectrum analysis gave almost the same figure (19 out of 48: 39.5%) with regard to rhythms with τ=24hr indicating that with one exception (subject JFL) rhythms were internally desynchronized including differences τ between right and left hand grip strength rhythms for three subjects. Results suggest: (a) a physiologic synchronization of circadian rhythms may be a predictor of good performance; (b) however, internal desynchronization as shown previously may be a trivial phenomenon and thus does not imply in itself alterations of either health or performance; (c) chronobiologic methods should be recommended for a better understanding of changes in performance by those participating in competitive and other sports.     

Alteration of period and amplitude of circadian rhythms in shift workers. With special reference to temperature, right and left hand grip strength

48 male shift workers in various industries volunteered to document circadian rhythms in sleeping and working, oral temperature, grip strength of both hands, peak expiratory flow and heart rate. All physiological variables were self-measured 4 to 5 times a day for 2 to 4 weeks. Individual time series were analyzed according to several statistical methods (power spectrum, cosinor, chi squares, ANOVA, correlation, etc.) in order to estimate rhythm parameters such as circadian period (tau) and amplitude (A), and to evaluate subgroup differences with regard to tolerance to shift work, age, duration of shift work, speed of rotation and type of industry. The present study confirms for oral temperature and extends to other variables (grip strength of both hands, heart rate) that intolerance to shift work is frequently associated with both internal desynchronization and small circadian amplitude. The internal desynchronization among several circadian rhythms supports the hypothesis that these latter are driven by several oscillators. Many differences were observed between circadian rhythms in right and left hand grip strength: circadian tau in oral temperature was correlated with that in the grip strength of the dominant hand but not with that of the other hand; changes in tau s of the non-dominant hand were age-related but did not correlate with temperature tau; only the circadian A of the non-dominant hand was associated with a desynchronization. Thus, circadian rhythms in oral temperature and dominant hand grip strength may be driven by the same oscillator while that of the non-dominant hand may be governed by a different one. Internal desynchronization between both hand grip rhythms as well as desynchronization of performance rhythms reported by others provide indirect evidence that circadian oscillator(s) may be located in the human cerebral cortex.     

Interindividual differences in a set of biological rhythms documented during the high arctic summer (79 degrees N) in three healthy subjects

The High Arctic summer with its permanent sunlight provides a situation in which one of the natural synchronizers, the light-dark alternation, is minimal. During the summers of 1981 and 1982 three healthy right-handed geographers who were performing field studies in Svalbard as part of their own research volunteered to document, 4-6 times per 24 hr for respectively 63,141 and 147 days, a set of circadian rhythms: self-rated fatigue, oral temperature, grip strength of both hands, heart rate and times of awakening and retiring. Tests were performed before departure from France, in Svalbard (79°IN latitude) where their daily activities were often strenuous, and after returning to France. Time series were treated individually according to three methods: display of data as a function of time, cosinor analyses to quantify rhythm parameters, and spectral analyses to estimate component periods of rhythms. Circadian parameters such as period and acrophase of activity-rest, oral temperature and fatigue rhythms were not altered. On the other hand, the circadian rhythm in grip strength was altered: the period differed from 24 hr in one subject, while grip strength acrophase of the left, but not the right, hand of the other two subjects was phase shifted during the sojourn in Svalbard. A prominent circahemidian (about 12 hr) rhythm was observed in two subjects for their heart rate in Svalbard, while a prominent circadian rhythm (differing from exactly 24 hr) was observed in France associated with a small circahemidian component.     

Interindividual Differences in a Set of Biological Rhythms Documented During the High Arctic Summer (79°N) In Three Healthy Subjects

The High Arctic summer with its permanent sunlight provides a situation in which one of the natural synchronizers, the light-dark alternation, is minimal. During the summers of 1981 and 1982 three healthy right-handed geographers who were performing field studies in Svalbard as part of their own research volunteered to document, 4–6 times per 24 hr for respectively 63,141 and 147 days, a set of circadian rhythms: self-rated fatigue, oral temperature, grip strength of both hands, heart rate and times of awakening and retiring. Tests were performed before departure from France, in Svalbard (79°IN latitude) where their daily activities were often strenuous, and after returning to France. Time series were treated individually according to three methods: display of data as a function of time, cosinor analyses to quantify rhythm parameters, and spectral analyses to estimate component periods of rhythms. Circadian parameters such as period and acrophase of activity-rest, oral temperature and fatigue rhythms were not altered. On the other hand, the circadian rhythm in grip strength was altered: the period differed from 24 hr in one subject, while grip strength acrophase of the left, but not the right, hand of the other two subjects was phase shifted during the sojourn in Svalbard. A prominent circahemidian (about 12 hr) rhythm was observed in two subjects for their heart rate in Svalbard, while a prominent circadian rhythm (differing from exactly 24 hr) was observed in France associated with a small circahemidian component.     

Circadian rhythm of the grip strength of the right and left hands: desynchronization in some shift workers

38 male shift workers and former shift workers volunteered to self-measure 4 to 8 times/24 hrs. their oral temperature (OT) as well as right and left hand grip strength (HGS) best performance during a 16 to 30 day span. Time series were analyzed individually according to two methods: day by day circadian acrophase drift and power spectrum. Mainly, but not exclusively, subjects with poor tolerance to shift work exhibited an internal desynchronization with a circadian period tau different from 24 hrs. which was the case for OT as well as right and left HGS; each could be different in tau between one another and from 24 hrs. These results suggest that oscillatory systems may be influenced by the neocortex apparently with difference between right and left side.     

Internal desynchronization of circadian rhythms and tolerance of shift work

Fifteen male subjects including 12 shift workers (oil refinery operators) volunteered to document circadian rhythms in sleep-wake, grip strength of both hands, peak expiratory flow, heart rate, self-rated drowsiness, fatigue and attention. Each of these variables was measured 4 to 6 times/day for 2 to 3 weeks. In addition, both axillary temperature (with a shielded probe) and wrist activity were almost continuously recorded at 15 min intervals during the same time span. Individual time series were analyzed according to several statistical methods (power spectrum, cosinor, chi 2, etc.), in order to estimate the prominent circadian period tau and to evaluate both individual and subgroup differences with regard to tolerance of shift work, age, duration of shift work. The present study confirms for continuously recorded temperature and wrist activity, grip strength of both hands, heart rate and peak expiratory flow that intolerance of shift work is frequently associated with an internal desynchronization. However, this conclusion cannot be extended to circadian rhythms in self-rated drowsiness, fatigue and attention. The internal desynchronization among several circadian rhythms supports the hypothesis that these latter are driven by several oscillators, with presumable differences between right and left hemispheres as suggested by unequal values of tau in rhythms of both hand grip strength. Since an internal desynchronization can be observed in tolerant shift workers having no complaint, it is likely that symptoms of intolerance are related to the subject's sensitivity to internal desynchronization rather than to the desynchronization itself.     

Effects of 24-Hour Shift Work with Nighttin Napping on Circadian Rhythm Characteristi in Ambulance Personnel

Forty-two ambulance personnel engaged in a 24-h shift system participated in a chronobiological field study to study the effects of 24-h shift work on circadian rhythm characteristics. Autorhythmometry of circadian rhythms of oral temperature, right and left grip strengths, and heart rate plus subjective assessment of drowsiness, fatigue, and attention was performed every ～ 4 h except during sleep for 7 days. Cosinor and power spectral analyses were applied to the longitudinal data of each individual. Changes in circadian period different from 24 h of oral temperature, grip strengths, and heart rate plus subjective drowsiness, fatigue, and attention were observed in ambulance personnel. The incidence of circadian periodicity different from 24 h in oral temperature and right and left grip strength was 28.6%, 35.7%, and 47.6%, respectively. The incidence was relatively lower than that of shift workers engaged in a discontinuous 8-h shift system we reported on previously. Working conditions allowing ambulance personnel to nap when not called for emergency (for > 4 h) might contribute to a stabilization of circadian rhythms. Furthermore, long nighttime ambulance service amounting to >100 min was significantly associated with a high incidence of at least one prominent circadian period among oral temperature and right and left grip strength rhythms different from 24 h. In conclusion, 24-h shift work altered the characteristics of circadian rhythms of ambulance personnel; nighttime naps seemed to have a favorable effect on averting changes in circadian rhythms.     

Comparison of grip strength and isomeric endurance between the right and left hands of men and their relationship with age and other physical parameters.

Maximum handgrip strength and endurance of fatiguing isometric handgrip muscle contraction at 40% of maximum voluntary contraction of the dominant hand were assessed separately for both right and left hands of 99 right-handed men aged 7-73 years. Subjects below 10 years (n = 6) could not follow up the endurance test methods and were excluded. The relationship of handgrip strength and endurance with age and other physical parameters was also assessed. Maximum grip strength and endurance of fatiguing submaximal contraction of the right hand were significantly greater than that of the left hand for most age groups. Grip strength was positively correlated with age from 7-19 years (r = 0.94 for right and r = 0.89 for left) and was negatively correlated with age from 20-73 years (r = -0.74 right and r = -0.69 left). Grip strength was positively correlated with the weight (r = 0.86 right and r = 0.87 left), height (r = 0.88 right and r = 0.87 left) and body surface area (r = 0.9 for both) of the subjects. Endurance of contraction of both hands did not show any relationship with age, different physical parameters or grip strength of the subjects.     

Age-related differences in corticospinal control during functional isometric contractions in left and right hands.

The purpose of the study was to examine age-related differences in electromyographic (EMG) responses to transcranial magnetic stimulation (TMS) during functional isometric contractions in left and right hands. EMG responses were recorded from the first dorsal interosseus muscle following TMS in 10 young (26.6 +/- 1.3 yr) and 10 old (67.6 +/- 2.3 yr) right-handed subjects. Muscle evoked potentials (MEPs) and silent-period durations were obtained in the left and right hands during index finger abduction, a precision grip, a power grip, and a scissor grip, while EMG was held constant at 5% of maximum. For all tasks, MEP area was 30% (P < 0.001) lower in the left hand of old compared with young subjects, whereas there was no age difference in the right hand. The duration of the EMG silent period was 14% (P < 0.001) shorter in old (150.3 +/- 2.9 ms) compared with young (173.9 +/- 3.0 ms) subjects, and the age differences were accentuated in the left hand (19% shorter, P < 0.001). For all subjects, the largest MEP area (10-12% larger) and longest EMG silent period (8-19 ms longer) were observed for the scissor grip compared with the other three tasks, and the largest task-dependent change in these variables was observed in the right hand of older adults. These differences in corticospinal control in the left and right hands of older adults may reflect neural adaptations that occur throughout a lifetime of preferential hand use for skilled (dominant) and unskilled (nondominant) motor tasks.     

Ultradian, Circadian and Circannual Rhythms of Blood Glucose and Injected Insulins Documented in Six Self-Controlled Adult Diabetics

The aim of the study was to document during one to two years individual rhythmic patterns in blood glucose and injected insulin in self-controlled insulin dependent (C-peptide negative) diabetics with home blood glucose monitoring. Two females and four males with diurnal activity from 0700 to 2300 self-determined their blood glucose three to six times a day over a period of 12-27 months. Circadian and ultradian rhythms were analysed for each subject on a monthly basis to document annual rhythms. Blood glucose (BG) estimated circadian acrophases were located between 2200 and 0300 for all patients and months with few exceptions. A correlation was found between circadian mesors and amplitudes of BG in four subjects. Annual changes in BG were validated for each subject with large interindividual differences in peak times. The individual mean of injected insulin (II) varied from 40 to 80 iU with annual changes validated for each subject. A group pattern was observed with a peak time either in the autumn (four patients) or in the summer (two patients). A circadian rhythm of II was detected in almost all monthly means and for all patients. Locations of computed peak time φ of II exhibited a great stability for a given individual but large interindividual differences. Thus the rather constant φ location of BG for all subjects contrasted with interindividual differences in φ locations of II. These results suggest that rhythmic changes in BG and II should be recognized when forming a realistic strategy for timing and dosing time(s) of insulin.     

Circadian rhythms in human performance efficiency under free-running conditions

A series of four experiments is described in which two men and two women (age range 22-79 years) were allowed to 'free-run' under conditions of isolation from all time cues. The circadian rhythms of performance efficiency were studied for three tasks: serial search, verbal reasoning, and manual dexterity (left and right hand versions). Strong evidence was obtained to suggest that different tasks are under the control of different circadian oscillators. The circadian rhythm in verbal reasoning performance tends to run at a shorter circadian period than that in more simple repetitive tasks, and slight differences in circadian behavior can occur between left- and right-handed dexterity.     

Internal desynchronization of circadian rhythms and tolerance to shift work

Intolerance to shift work may result from individual susceptibility to an internal desynchronization. Some shift workers (SW) who show desynchronization of their circadian rhythms (e.g., sleep-wake, body temperature, and grip strength of both hands) exhibit symptoms of SW intolerance, such as sleep alteration, persistent fatigue, sleep medication dependence, and mood disturbances, including depression. Existing time series data previously collected from 48 male Caucasian French SW were reanalyzed specifically to test the hypothesis that internal synchronization of circadian rhythms is associated with SW intolerance and symptoms. The entry of the subjects into the study was randomized. Three groups were formed thereafter: SW with good tolerance (n=14); SW with poor tolerance, as evident by medical complaints for at least one year (n=19); and former SW (n=15) with very poor tolerance and who had been discharged from night work for at 1.5 yr span but who were symptom-free at the time of the study. Individual and longitudinal time series of selected variables (self-recorded sleep-wake data using a sleep log, self-measured grip strength of both hands using a Colin Gentile dynamometer, and oral temperature using a clinical thermometer) were gathered for at least 15 days, including during one or two night shifts. Measurements were performed 4-5 times/24 h. Power spectra used to quantify the prominent period (tau) and t-test, chi square, and correlation coefficient were used as statistical tools. The mean (+/-SEM) age of SW with good tolerance was greater than that of SW with poor tolerance (44.9+/-2.1 yrs vs. 40.1+/-2.6 yrs, p<.001) and of former SW discharged from night work (very poor tolerance; 33.4+/-1.7, p<.001). The shift-work duration (yrs) was longer in SW with good than poor tolerance (19.9+/-2.2 yrs vs. 15.7+/-2.2; p<0.002) and former SW (10.7+/-1.2; p<.0001). The correlation between subject age and shift-work duration was stronger in tolerant SW (r=0.97, p<.0001) than in non-tolerant SW (r=0.80, p<0.001) and greater than that of former SW (r=0.72, p<.01). The mean sleep-wake rhythm tau was 24 h for all 48 subjects. The number of desynchronized circadian rhythms (tau differing from 24 h) was greater in non-tolerant than in tolerant SW (chi square=38.9, p<.0001). In Former SW (i.e., 15 individuals assessed in follow-up studies done 1.5 to 20 yrs after return to day work), both symptoms of intolerance and internal desynchronization were reduced or absent. The results suggest that non-tolerant SW are particularly sensitive to the internal desynchronization of their circadian time organization.     

Circadian rhythms in healthy aging--effects downstream from the pacemaker

Using both previously published findings and entirely new data, we present evidence in support of the argument that the circadian dysfunction of advancing age in the healthy human is primarily one of failing to transduce the circadian signal from the circadian timing system (CTS) to rhythms “downstream” from the pacemaker rather than one of failing to generate the circadian signal itself. Two downstream rhythms are considered: subjective alertness and objective performance. For subjective alertness, we show that in both normal nychthemeral (24h routine, sleeping at night) and unmasking (36h of constant wakeful bed rest) conditions, advancing age, especially in men, leads to flattening of subjective alertness rhythms, even when circadian temperature rhythms are relatively robust. For objective performance, an unmasking experiment involving manual dexterity, visual search, and visual vigilance tasks was used to demonstrate that the relationship between temperature and performance is strong in the young, but not in older subjects (and especially not in older men). (Chronobiology International, 17(3), 355-368, 2000)     

Right- and left-brain hemisphere. Rhythm in reaction time to light signals is task-load-dependent: age,gender, and handgrip strength rhythm comparisons

In healthy mature subjects simple reaction time (SRT) to a single light signal (an easy task) is associated with a prominent rhythm with tau = 24 h of dominant (DH) as well as nondominant (NDH) hand performance, while three-choice reaction time (CRT), a complex task, is associated with tau = 24 h of the DH but tau < 24 h of the NDH. The aims of the study were to assess the influence of age and gender on the difference in tau of the NDH and DH, as it relates to the corresponding cortical hemisphere of the brain, in comparison to the rhythm in handgrip strength. Healthy subjects, 9 (5 M and 4 F) adolescents 10-16 yr of age and 15 (8 M and 7 F) adults 18-67 yr of age, active between 08:00 +/- 1 h and 23:00 +/- 1:30 h and free of alcohol, tobacco, and drug consumption volunteered. Data were gathered longitudinally at home and work 4-7 times daily for 11-20 d. At each test time the following variables were assessed: grip strength of both hands (Dynamometer: Colin-Gentile, Paris, France); single reaction time to a yellow signal (SRT); and CRT to randomized yellow, red, or green signal series with varying instruction from test to test (Psycholog-24: Biophyderm, France). Rhythms in the performance in SRT, CRT, and handgrip strength of both DH and NDH were explored. The sleep-wake rhythm was assessed by sleep-logs, and in a subset of 14 subjects it was also assessed by wrist actigraphy (Mini-Motionlogger: AMI, Ardsley NY). Exploration of the prominent period tau of time series was achieved by a special power spectra analysis for unequally spaced data. Cosinor analysis was used to quantify the rhythm amplitude A and rhythm-adjusted mean M of the power spectral analysis determined trial tau. A 24h sleep-wake rhythm was detected in almost all cases. In adults, a prominent tau of 24 h characterized the performance of the easy task by both the DH and NDH. In adults a prominent tau of 24 h was also detected in the complex CRT task performed by the DH, but for the NDH the tau was < 24 h. This phenomenon was not gender-related but was age-related since it was seldom observed in adolescent subjects. Hand-side differences in the grip strength rhythms in the same individuals were detected, the tau being ultradian rather than circadian in adolescent subjects while in mature subjects the tau frequently differed from that of the rhythm in CRT. These findings further support the hypothesis that functional biological clocks exist in both the left and right hemispheres of the human cortex.     

The genetic background of individual variations of circadian-rhythm periods in healthy human adults.

As a group phenomenon, human variables exhibit a rhythm with a period (tau) equal to 24 h. However, healthy human adults may differ from one another with regard to the persistence of the 24-h periods of a set of variables' rhythms within a given individual. Such an internal desynchronization (or individual circadian dyschronism) was documented during isolation experiments without time cues, both in the present study involving 78 male shift workers and in 20 males and 19 females living in a natural setting. Circadian rhythms of sleep-wake cycles, oral temperature, grip strength of both hands, and heart rate were recorded, and power-spectra analyses of individual time series of about 15 days were used to quantify the rhythm period of each variable. The period of the sleep-wake cycle seldom differed from 24 h, while rhythm periods of the other variables exhibited a trimodal distribution (tau = 24 h, tau > 24 h, tau < 24 h). Among the temperature rhythm periods which were either < 24 h or > 24 h, none was detected between 23.2 and 24 h or between 24 and 24.8 h. Furthermore, the deviations from the 24-h period were predominantly grouped in multiples of +/- 0.8 h. Similar results were obtained when the rhythm periods of hand grip strength were analyzed (for each hand separately). In addition, the distribution of grip strength rhythm periods of the left hand exhibited a gender-related difference. These results suggested the presence of genetically controlled variability. Consequently, the distribution pattern of the periods was analyzed to elucidate its compatibility with a genetic control consisting of either a two-allele system, a multiple-allele system, or a polygenic system. The analysis resulted in structuring a model which integrates the function of a constitutive (essential) gene which produces the exact 24-h period (the Dian domain) with a set of (inducible) polygenes, the alleles of which, contribute identical time entities to the period. The time entities which affected the rhythm periods of the variables examined were in the magnitude of +/- 0.8 h. Such an assembly of genes may create periods ranging from 20 to 28 h (the Circadian domain). The model was termed by us "The Dian-Circadian Model." This model can also be used to explain the beat phenomena in biological rhythms, the presence of 7-d and 30-d periods, and interindividual differences in sensitivity of rhythm characteristics (phase shifts, synchronization, etc.) to external (and environmental) factors.     

Age-related changes in hand grip strength among rural and urban Haryanvi Jat females

The aim of this study was to investigate the age-related changes in hand grip strength (both right and left hand) among rural and urban Jat females and its correlation with various anthropometric variables. The present cross-sectional study was based on a sample of 600 Jat females (rural=300, urban=300), ranging in age from 40 to 70 years. Findings indicated that rural and urban Jat females exhibit an age-related decline in the mean values of hand grip strength (both right and left hand). The comparison suggested that rural Jat females had significantly higher overall mean value of hand grip strength (kg) (right hand=20.35 vs 18.87, p<0.001 and left hand=17.05 vs 15.68, p<0.001) than their urban counterparts. Mean values for right hand grip strength were found to be higher than left hand grip strength in rural as well as urban Jat females. Significant and positive correlation coefficients exist between hand grip strength (both right and left hand) and various anthropometric variables (height, weight, body mass index, biceps and triceps skinfold thickness) in both rural and urban Jat females. An age-related decline in manual functioning of rural and urban Jat females may be attributed to behavioral factors (reduced physical activity, sedentary lifestyle) and poor nutrition (deteriorating quality and quantity of food intake with increasing age) associated with the normal aging process.     

Ultradian and asymmetric rhythms of hemispheric processing speed

Daily changes in cognitive performance have been documented, both in time of day/effect paradigm studies and in time-isolation studies. However, in both types of studies, phenomena such as the “post-lunch dip” have been found that were difficult to explain in terms of theoretical backgrounds. These phenomena may suggest ultradian rhythms in cognitive performance. A number of studies have also shown ultradian and asymmetric rhythms in activity indices of the brain hemispheres. The aim of this study was to test three hypotheses: the first two assumed that there is a significant ultradian frequency in a component of the endogenous rhythm of processing speed, and the third assumed that the ultradian endogenous rhythms of the processing speed (encoding and recognition) of stimuli addressed to the left brain hemisphere differ in period length from those addressed to right hemisphere. During a 24 h constant-routine experiment, the memory performance of 30 participants was measured eight times (every 2.5-3 h), starting at 06:30 h. Parallel sets of words and pictures were shown to subjects in a random order in either the left or the right visual field on a computer screen. The participants pressed one of two buttons in response to the picture or word, or when answering a question concerning the meaning of a presented stimulus. Cosinor analysis was applied to individual time series data. Two significant ultradian components were found in a majority of the time series. Dominant periods were analyzed using three factor ANOVA. The results showed an asymmetry between both hemispheres in the frequency of ultradian rhythms in encoding speed.     

Light Treatment for NASA Shiftworkers

Intense artificial light can phase-shift circadian rhythms and improve performance, sleep, and well-being during shiftwork simulations. In real shiftworkers, however, exposure to sunlight and other time cues may decrease the efficacy of light treatment, and occupational and family responsibilities may make it impractical. With these considerations in mind, we designed and tested light-treatment protocols for NASA personnel who worked on shifted schedules during two Space Shuttle missions. During the prelaunch week, treatment subjects self-administered light of ∼10,000 lux at times of day that phase-delay circadian rhythms. Treatment continued during the missions and for several days afterward. No treatment was administered to subjects in the control group. Treatment subjects reported better sleep, performance, and physical and emotional well-being than control subjects and rated the treatment as highly effective for promoting adjustment to their work schedules. Light treatment is both feasible and beneficial for NASA personnel who must work on shifted schedules during Space Shuttle missions.     

Circadian Determinants of the Postlunch Dip in Performance

This study compared rectal temperature rhythms in groups of subjects who either did (n = 5) or did not (n - 7) show a clear postlunch dip in performance at a monotonous (25-30 min) vigilance task. Performance was tested every 2h in a standardized routine with lunch replaced by hourly liquid food supplements. Those showing the postlunch performance dip had a higher amplitude and later peaking 12h component to their rectal temperature rhythm that those who did not, resulting in flat, rather than rising, temperatures over the 10:00-15:00 time interval. The effect could not be explained by intergroup differences in prior sleep, morningness, or gender, although there was a trend (p = 0.09) for the “dip” group to be slightly younger (21.8y vs. 24.2y). The postlunch dip appears to be an endogenous phenomenon individually determined, but related to the strength of the (12h) harmonic of the circadian system.