Ultradian and circadian CO2 emission variations in nocturnal and diurnal animals exposed to a light stimulus

1. Carbon dioxide emission (VCO2) has been continuously recorded in three laboratory animal species (Sprague-Dawley rats, Japanese quail, Hartley guinea-pigs) which differ by their nocturnal and diurnal activities. A 100 lux stimulus has been delivered at various time intervals. 2. A regular alternation of 12, 3 or 1.5 hr light (L) and darkness (D) gives VCO2 circadian and ultradian rhythms of 24, 6 or 3 hr periods, respectively, in quail and rats. 3. Such circadian and ultradian LD rhythms are not induced in all guinea-pigs. 4. The amplitudes of the VCO2 responses are greatest at D----L when the animals have a maximum diurnal activity and at L----D when their maximum activity is nocturnal. 5. Interactions between circadian and ultradian rhythms are seen in all LD experiments, as well as in continuous light (LL) or continuous dark (DD). 6. No more well-marked or even inverted VCO2 responses to the light stimuli may occur after several days of exposure to these LD alternations.     

Ultradian and circadian compartmentalization of respiratory and metabolic exchanges in small laboratory vertebrates

Carbon dioxide emission (VCO2) taken as an index of respiratory and metabolic exchanges, was continuously recorded during 4-30 consecutive days in 100 quail, 87 chicks, 347 rats, 665 mice and 70 guinea-pigs which were under controlled environmental parameters. Harmonic analysis, fast Fourier transform, chi-square periodograms, peak and trough intervals were computed with VCO2 values obtained with CO2 concentrations sampled every 20 min on the CO2 recordings. In LD 12:12 alternation, circadian rhythms were observed in all quail, chicks, rats and mice, but only in 80% of the guinea-pigs. Ultradian VCO2 rhythms, with periods which show statistically significant interspecies differences, were assessed. For each of the 5 species these computed periods, which were the same in LL and DD, were: 1.17 h for quail and chickens, 1.25 h for rats, 1.50 h for mice and 1.0 h for guinea-pigs. In LD 12:12 these periods were different during L and D in quail, chicks, rats and mice, but not in guinea-pigs. The amplitudes of these ultradian variations were, according to the species, 10-20% of their mean VCO2 levels. These ultradian rhythms persist in the absence (or masking) of circadian rhythms, e.g. in LD 12:12 in 20% of guinea-pigs and in LL in 87% of Japanese quail and in 23% of Sprague-Dawley rats. Moreover, these ultradian rhythms persist during starvation, locomotor activity restraint and ageing. These ultradian VCO2 cycles which are related to rest-activity variations appear to be basic physiological rhythms with a genetic origin.     

Societal synchronization in groups of rats or quail submitted to various lighting regimens

Groups of rats or of quail that had been previously synchronized in a light (L = 100 lux) dark (D) phase opposition (PO = LD and DL) were placed together in a L12:D12 or D12:L12 alternation or in continuous light (LL) or continuous darkness (DD). Emission of carbon dioxide (VCO2) which was continuously recorded in groups of individuals placed in respiratory chambers under controlled environmental conditions allows an index of their overall respiratory and metabolic exchanges to be found. In PO animals placed in LD or DL, the VCO2 circadian light dark synchronization comes back less quickly in rats than in quail, and the VCO2 variations at the light dark transitions (L-D and D-L) remain unchanged in rats, but are modified in quail. When PO animals are placed for 18 days in LL or DD, respiratory circadian rhythms disappear except in the grouped rats where they reappear after 4-5 days in DD.     

Periodic analysis of ultradian (40 min less than tau less than 24h) respiratory variations in laboratory vertebrates of various circadian activities

Continuous recordings of respiratory gas exchanges of various laboratory endotherm vertebrate species, which have either a nocturnal (mouse, rat) or diurnal (monkey, quail, chicken) or equivocal (guinea-pig) maximal activity, kept under controlled environmental conditions of temperature, humidity, ventilation and provided with food and water ad libitum, show ultradian oscillations of mean and low frequencies (1 less than f less than 35 c.day-1). Harmonic analysis was used to assess periodic or random ultradian variations and to compute amplitudes and phases of these oscillations when these vertebrates were submitted to a light (100 lx) and dark circadian alteration (LD 12:12). Spectral analysis shows that a 100-lx continuous illumination or continuous darkness decreases circadian respiratory rhythms and increases these ultradian respiratory oscillations.     

Harmonic analysis of ultradian respiratory rhythms in four small laboratory vertebrates lit in LD12:12

Fourier harmonic analysis has been applied to 20-min samples of VCO2, recorded every 12 sec over several (7-12) days, in one mouse, one rat, one guinea-pig, or one quail, maintained in controlled conditions of temperature, humidity and ventilation, and lit in LD12:12 (100 lux). The harmonic computations, during L and D of ultradian periods (1.2 hr less than tau less than 12 hr) evidence statistically significant differences in amplitude and phase between these four small laboratory species. These periodic respiratory differences correspond to discrepancies in their diurnal and nocturnal activities, and in their responses to light and dark.     

The internal synchronization of five circadian functions of the rabbit

Five different physiological functions of the rabbit (hard faeces and urine excretion, food and water intake and locomotor activity) were registered during LD 12:12 and during continuous light conditions (LL).

(1) In LD 12:12 a strong synchronization of the five parameters existed. The minima of all functions consistently occurred during the hours of light. The nocturnal percentage of overall 24-hr events was increased significantly in 'hard faeces excretion' (66±8 (S.D.) %), 'water intake' (64±15 (S.D.) %) and 'urine excretion' (58±10 (S.D.) %). The nocturnal percentage of locomotor activity was significantly increased during the dark-hours in 9 out of 14 animals. In the other five individuals prominent peaks were present even during the photoperiod. On the average of all 14 animals 5S±13 (S.D.) % of the 24 hr events of locomotor activity occurred during the night. Despite a trough during the cessation of hard faeces excretion the events of food intake were not elevated significantly during the dark hours.

(2) During LL the synchronization of the five functions within each animal persisted during the complete 90-day LL period. A free-running circadian rhythm with-: = 24.8±0.5 (S.D.) hr was present in the four rabbits kept in LL conditions within 5-16 days after the withdrawal of the zeitgeber.

(3) In addition to the circadian period the power spectrum analysis of data obtained during LD 12:12 revealed significant ultradian periods of an average period length of 11,6 hr (hard faeces and urine excretion), 8 hr (food and water intake, locomotor activity) and 4 hr (food intake, locomotor activity). During the free-run ultradian periods of 8 and 3.2-4.2 hr were significant in almost all parameters.

(4) During LL the level of locomotor activity was reduced for 13±16 (S.D.) %, the events of food intake were increased for 17±12 (S.D.) %.

(5) The reinserted LD 12:12 zeitgeber re-entrained the circadian rhythms within 25-45 days.

(6) These results provided evidence of a predominant nocturnality of the rabbits under investigation.     

Ultradian respiratory rhythms in groups of Sprague-Dawley rats of both sexes

Spectral analysis of the carbon dioxide emitted in controlled environmental conditions (temperature: 20-21 degrees C; humidity: 50-60%; lighting: 150 lux in LD12-12; background noise: 70 dB), by groups of 14 month old Sprague Dawleys, shows ultradian respiratory rates with periods of a 107-72 minute range for males and 107-45 minute for females. Moreover, VCO2 related to body weight is higher in females than in males.     

The Correlation Between Activity, Blood Pressure and Heart Rate in Freely Mobile Sprague-Dawley and Transgenic Rats

Systolic (SBP) and diastolic blood pressure (DBP), heart rate (HR) and locomotor activity have been measured at 1-min intervals for 24 h in Sprague-Dawley (N = 5) and for 2x24 h in transgenic hypertensive (N = 4) rats. The animals were freely mobile and entrained to a 12:12 LD cycle (lights on at 0700). The endogenous circadian component of the cardiovascular variables was removed from the raw data, and then correlations between activity and the residual component (raw data minus the endogenous component) of SBP, DBP and HR were calculated. This calculation was performed twice, in the mid-light and mid-dark phases. We have investigated if the mean size of the correlation coefficients depended on cardiovascular variable (SBP, DBP or HR), phase (D or L) or strain (Sprague-Dawley, SPD, or Transgenic, TG, rats). Nearly all correlations were positive and ANOVA's showed a significant effect of cardiovascular variable for both strains, with correlations for HR being significantly higher than those for SBP and DBP. The mean correlations in the SPD strain were significantly higher than in the TG strain for variables SBP and DBP, but not for HR. The correlations between activity and blood pressure were more marked for SPD rats in the light (inactive) than dark (active) phase. Both strains showed ultradian rhythms in all variables, particularly in the light phase. If the analysis was repeated using deviations of the cardiovascular variables from a 1-h moving average rather than the endogenous circadian component, then the results were very similar. The results are discussed in terms of the links between the rhythms of activity and cardiovascular variables, with particular reference to differences between the two strains.     

The Correlation Between Activity,Blood Pressure and Heart Rate in Freely Mobile Sprague-Dawley and Transgenic Rats

Systolic (SBP) and diastolic blood pressure (DBP), heart rate (HR) and locomotor activity have been measured at 1-min intervals for 24 h in Sprague-Dawley (N = 5) and for 2x24 h in transgenic hypertensive (N = 4) rats. The animals were freely mobile and entrained to a 12:12 LD cycle (lights on at 0700). The endogenous circadian component of the cardiovascular variables was removed from the raw data, and then correlations between activity and the residual component (raw data minus the endogenous component) of SBP, DBP and HR were calculated. This calculation was performed twice, in the mid-light and mid-dark phases. We have investigated if the mean size of the correlation coefficients depended on cardiovascular variable (SBP, DBP or HR), phase (D or L) or strain (Sprague-Dawley, SPD, or Transgenic, TG, rats). Nearly all correlations were positive and ANOVA's showed a significant effect of cardiovascular variable for both strains, with correlations for HR being significantly higher than those for SBP and DBP. The mean correlations in the SPD strain were significantly higher than in the TG strain for variables SBP and DBP, but not for HR. The correlations between activity and blood pressure were more marked for SPD rats in the light (inactive) than dark (active) phase. Both strains showed ultradian rhythms in all variables, particularly in the light phase. If the analysis was repeated using deviations of the cardiovascular variables from a 1-h moving average rather than the endogenous circadian component, then the results were very similar. The results are discussed in terms of the links between the rhythms of activity and cardiovascular variables, with particular reference to differences between the two strains.     

Perspectives in chronobiology of air pollution

In a series of experiments, male and female Sprague Dawley rats, kept in light (L) from 06(00) to 18(00) alternating with darkness (LD 12:12) inhaled different concentrations of carbon monoxide (50-1,700 ppm) at each of two test times, 12 h apart. A decrease in flow of CO2 (VCO2) resulting from CO inhalation was greater in the active dark (D) than resting light (L) span. Experimental hypoxic mortality of male and female mice also shows circadian variations, being greater in the D than in the L span. Moreover, a difference of mortality was observed betwen hypoxic exposures performed at 12(00) (in LD or DL) and hypoxic exposures performed at 00(00) (in LD or DL). Such results await tests of any extent to which they model responses of human beings to air pollution. In human beings any external environmental circadian, circaseptan and circannual variations in air pollution as such may serve to variable extent as socioeconomic synchronizers of innate rhythms with a corresponding frequency, rather than as solely generators of time patterns in any physiopathologic response to air pollution.     

Internal coupling of five functions of rabbits exhibiting a bimodal circadian rhythm

Abstract

By means of a microcomputer‐assisted, electronic recording system five physiological, circadian rhythms of the rabbit were monitored: locomotor activity, hard faeces excretion, food intake, urine excretion and water intake.

During 120 days of continuous light conditions (30 lx) the animals exhibited a free‐running circadian rhythm. After the fading out of aftereffects of the preceding light: dark schedule on day 51 ± 11 the animals ran free with an individually distinct period length of > 24.0 h (τ: 24.48 ± 0.10 (SD) h). Spectral analysis of coherent data of 50–84 days showed that in addition to the circadian period persistent ultradian periods of 6.1, 8.2 and 12.3 h were present. Within each individual the five functions proved to be tightly coupled during the free‐run, during the time of reentrainment and when entrained with the LD 12:12. While during LD 12:12 the animals exhibited a bimodal rhythm, during the free‐run the rhythm was unimodal in all five functions. In one animal a “splitting”; of the free‐running period occurred. Both components ran free with different period length. They fused again after 38 days. The “splitting”; was reflected in all five functions of this animal.

The behavioural characteristics of meal duration and ‐frequency, duration of activity and ‐intervals, of water intake and urination did not show significant differences during the conditions of LD 12:12 and LL.

The results support Pittendrigh's model of two systems of oscillators, selectively susceptible to the transitions of dark : light and light : dark. Our results suggest that in the rabbit the five functions are governed in common by both oscillator‐systems.     

Changes in light and dark periods affect the arylalkylamine N-acetyl transferase,melatonin activities and redox status in the head and hemolymph of nocturnal insect Spodoptera litura

The present study was conducted to describe the impact of circadian rhythm on melatonin levels and redox statusunder three photoperiods (12L:12D, 0L:24D, and 24L:0D) in head and hemolymph of Spodoptera litura. Melatonin is an powerful antioxidant and controls the reproduction of organisms. In this study, melatonin levels, Arylalkylamine N-acetyltransferase(AA-NAT), and antioxidant enzyme activities were analyzed. Results showed melatonin, AA-NAT levels in hemolymph were significantly (p < 0.05) higher during the dark period than during LL regime. HPLC chromatogram of the insect head and hemolymph showed 5 peaks while hemolymph showed 6 peaks in LD, and LLregimes. The day–night changes of melatonin increased the antioxidant enzymes (GST, CAT, POX) persisted in the insect hemolymph, but were suppressed by constant light. The present study leads us to speculate that synthesis and release of melatonin in the S.litura head occur as circadian rhythm and light has an inhibitory effect on melatonin synthesis.     

Characteristics of the circadian activity rhythm in common marmosets (Callithrix j. jacchus)

The circadian activity rhythm of the common marmoset, Callithrix j. jacchus was investigated by long-term recording of the locomotor activity of 15 individuals (5 males, 10 females) from 1.5 to 8 years old, both under constant illumination and under LD 12:12. The mean period of the spontaneous circadian rhythm was 23.2 ± 0.3 h. Neither sex-specific differences nor a systematic influence of light intensity on the spontaneous period were observed, but the period was dependent on the duration of the trial and on the age of the individual. Due to the short spontaneous period, in LD 12:12 there was a distinct advance of the activity phase with respect to the light time and a masking of the true onset of activity by the inhibitory direct effect of low light intensity during the dark time. After an 8 h delay shift of the LD 12:12, re-entrainment of the circadian activity rhythm required an average of 6.8 ± 0.7 days; the average re-entrainment time after an 8 h phase advance of the LD cycle was 8.6 ± 1.3 day. This directional effect is ascribed to characteristics of the phase-response curve. No ultradian components were observed, either in the LD-entrained or the free-running circadian activity rhythm.     

Dissociation of ultradian and circadian phenotypes in female and male siberian hamsters

Three experiments addressed whether pronounced alterations in the circadian system yielded concomitant changes in ultradian timing. Female Siberian hamsters were housed in a 16L:8D photoperiod after being subjected to a disruptive phase-shifting protocol that produced 3 distinct permanent circadian phenotypes: some hamsters entrained their circadian rhythms (CRs) with predominantly nocturnal locomotor activity (ENTR), others displayed free-running CRs (FR), and a third cohort was circadian arrhythmic (ARR). The period of the ultradian locomotor rhythm (UR) did not differ among the 3 circadian phenotypes; neuroendocrine generation of URs remains viable in the absence of coherent circadian organization and appears to be mediated by substrates functionally and anatomically distinct from those that generate CRs. Pronounced light-dark differences in several UR characteristics in ENTR hamsters were completely absent in circadian arrhythmic hamsters. The disruptive phase-shifting protocol may compromise direct visual input to ultradian oscillators but more likely indirectly affects URs by interrupting visual afference to the circadian system. Additional experiments documented that deuterium oxide and constant light, each of which substantially lengthened the period of free-running CRs, failed to change the period of concurrently monitored URs. The resistance of URs to deuteration contrasts with the slowing of virtually all other biological timing processes, including CRs. Considered together, the present results point to the existence of separable control mechanisms for generation of circadian and ultradian rhythms.     

MELANOPSIN AND CLOCK GENES: REGULATION BY LIGHT AND ENDOTHELIN IN THE ZEBRAFISH ZEM-2S CELL LINE

It is well known that clocks are present in brain regions other than the suprachiasmatic nucleus and in many peripheral tissues. In the teleost, Danio rerio, peripheral oscillators can be directly synchronized by light. Danio rerio ZEM-2S embryonic cells respond to light with differential growth: cells kept in constant light exhibited a strong inhibition of proliferation, whereas in cells kept in light:dark (LD) cycles (14L:10D and 10L:14D) or in constant darkness (DD), the doubling times were not statistically different. We demonstrated by RT-PCR followed by PCR that ZEM-2S cells express two melanopsins, Opn4x and Opn4m, and the six Cry genes. The presence of the protein OPN4x was demonstrated by immunocytochemistry. The pattern of temporal expression of the genes Opn4x, Per1, Cry1b, and Clock was studied in ZEM-2S cells kept for five days in 12L:12D or DD. In 12L:12D, the clock genes Per 1 and Cry1b exhibited robust circadian expression, while Opn4x and Clock expression seemed to vary in an ultradian pattern. Both Per1 and Cry1b genes had higher expression during the L phase; Clock gene had an increase in expression coincident with the D phase, and during the subjective night. In DD, the temporal variation of Per1 and Cry1b genes was greatly attenuated but not extinguished, and the higher expressions were shifted to the transition times between subjective day and night, demonstrating that Per and Cry1b were synchronized by the LD cycle. Clock and Opn4x kept the ultradian oscillation, but the rhythm was not statistically significant. As endothelins (ET) have been reported to be a potent stimulator of Per genes in rodents, we investigated the effect of endothelin on ZEM-2S cells, which express ETA receptors. Cells were kept in 12D:12L for five days, and then treated with 10^???11 to 10^???8M ET-1 for 24?h. ET-1 exhibited a biphasic effect on Opn4x expression. At 10^???11M, the hormone exerted a highly significant stimulation of Opn4x expression during the L phase and introduced a circadian oscillatory pattern. At 10^???10M, a significant increase was seen at ZT21 and ZT0 (i.e., at the end of the D phase and beginning of the L phase), whereas 10^???9 and 10^???8M ET-1 inhibited the expression of Opn4x at most ZTs. Clock expression was unaffected by 10^???8M ET-1; however, in the presence of lower concentrations, the expression was enhanced at some ZTs, strengthening the ultradian oscillation. ET-1 at 10^???11 and 10^???10M had no effect on Per1 circadian expression; however, 10^???9 and 10^???8M ET-1 reduced the amplitude of Per1 expression in the beginning of the L phase. ET-1 effects were less evident on Cry 1b. For both genes, the reduction in expression was not sufficient to abolish the circadian oscillatory pattern. Based on these results and data in the literature, a link between ET-1 stimulation of ETA receptors may be established by E4BP4 binding to the promoters and consequent inhibition of gene expression. (Author correspondence: amdlcast@ib.usp.br)     

PACEMAKER PHASE CONTROL VERSUS MASKING BY LIGHT: SETTING THE CIRCADIAN CHRONOTYPE IN DUAL OCTODON DEGUS

There are two main processes involved in the expression of circadian rhythmicity: entrainment and masking. Whereas the first operates via the central pacemaker to anticipate predictable environmental conditions, masking (mainly induced by light) functions as a direct modulator of the circadian output signal induced by nonpredictable events. The Chilean rodent Octodon degus presents both diurnal and nocturnal chronotypes when given free access to an exercise wheel. Two steady-entrainment phases and graded masking by light seem to generate the wide variability of chronotypes in this species. The aim of this study was to characterize the differential masking by light according to the individual chronotypes, their stability over time, and the influence of wheel running availability and ambient temperature upon the degus' nocturnality. To this end, diurnal and nocturnal degus were subjected to ultradian cycles (1:1-h light-dark [LD]), with and without wheel running availability, and under both normal and high diurnal ambient temperature cycles. The present results show that diurnal and nocturnal degus present a stable masking by light, each according to its respective chronotype. Thus, whereas diurnal animals increased their activity with light, in nocturnal degus light induced a sharp drop in wheel running activity. These two types of masking responses appeared not only when the animals were synchronized to the 12:12-h LD cycle, but also under ultradian cycles. Different masking effects persisted when wheel running was made unavailable and when the animals shifted their circadian activity patterns in response to ultradian cycles or to diurnal exposure to high temperatures. In conclusion, our results show that the positive and negative masking effects of light on diurnal and nocturnal degus, respectively, seem to occur independently of relative phase control by the central pacemaker or the negative masking induced by high environmental temperatures. (Author correspondence: angerol@um.es)     

The Effects of Various Lighting Schedules Upon the Orcadian Rhythms of 23 Liver or Brain Enzymes of C57Bl/6J Mice

The activities of 23 brain or liver enzymes were studied in 5–6 week old C57BL/6JNctr male and female mice that had been fed ad libitum and standardized for 2 weeks to either (1) 12 hr of light (0600-1800) alternating with 12 hr of darkness (1800-0600) (LD 12:12), (2) staggered sequences of 12 hr of light and 12 hr of dark (SLD 12:12) or (3) continuous illumination (LL 12:12) for 2 weeks. Mice in the LD 12:12 and LL 12:12 experiments were killed at 4 hr intervals along a 24-hr span in order to sample at six different circadian stages. Lighting schedules for mice in the SLD 12:12 experiment were organized such that six different circadian stages were sampled when all mice were killed at one time of day.

All 23 enzymes demonstrated a prominent circadian rhythm in at least one of the experiments. Moreover, about two-thirds of the enzymes in LD and SLD 12:12 had a statistically significant fit to a 24-hr cosine curve, while only one-third of the enzymes in LL 12:12 had significant fits to cosine curves. Peak activities of enzymes from mice in LD 12:12 were clustered at the time of transition from light to dark. This was also the trend for the activities of enzymes from mice in SLD 12:12, but resynchronization did not appear completed within the 2-week span. This, along with the observation that mesors (mean 24-hr activity) were reduced and amplitudes altered, indicated that the 2-week standardization period was not sufficient for some enzymes. Times of peak activities, mesors and amplitudes were affected for most enzymes from mice in the LL 12:12 environment. This suggests that individual mice became desynchronized from one another with respect to the original light-dark schedule and that rhythms were altered or lost because individual mice were free running with frequencies different from 24 hr.     

A circadian and an ultradian rhythm are both evident in root growth of rice

This paper presents evidence for the existence of both a circadian and an ultradian rhythm in the elongation growth of rice roots. Root elongation of rice (Oryza sativa) was recorded under dim green light by using a CCD camera connected to a computer. Four treatment conditions were set-up to investigate the existence of endogenous rhythms: 28 °C constant temperature and continuous dark (28 DD); 28 °C constant temperature and alternating light and dark (28 LD); 33 °C constant temperature and continuous dark (33 DD); and diurnal temperature change and alternating light and dark (DT-LD). The resulting spectral densities suggested the existence of periodicities of 20.4-25.2 h (circadian cycles) and 2.0-6.0 h (ultradian cycles) in each of the 4 treatments. The shorter ultradian cycles can be attributed to circumnutational growth of roots and/or to mucilage exudation. The average values across all the replicate data showed that the highest power spectral densities (PSDs) corresponded to root growth rhythms with periods of 22.9, 23.7, and 2.1 h for the 28 DD, 28 LD, and 33 DD treatments, respectively. Accumulation of PSD for each data set indicated that the periodicity was similar in both the 28 DD and 33 DD treatments. We conclude that a 23-h circadian and a 2-h ultradian rhythmicity exist in rice root elongation. Moreover, root elongation rates during the day were 1.08 and 1.44 times faster than those during the night for the 28 LD and DT-LD treatments, respectively.     

Multiple Component Analysis of Plasma Growth Hormone in Children with Standard and Short Stature

Growth hormone (GH) concentrations (in ng/ml) were determined by radioimmunoassay, in plasma obtained at about 3-hr intervals during a 24-hr sampling span, from 42 boys and 12 girls of short stature (2-4 standard deviations below their peer group mean), and 13 boys and 9 girls of standard stature. Subjects had 11.20 0.37 years of age at the time of study, and were living on a diurnal waking (∼07:30 to ∼22:30), nocturnal resting routine during sampling. Analysis of these data by single and population-mean cosinor methods as well as by analysis of variance revealed circadian and ultradian prominent components characterizing most groups. Accordingly, a multiple component analysis was undertaken for data of each group separately, as well as for all subjects. A comparison of circadian parameters indicates similar characteristics between short and standard children, whether one compares boys [P=0.674, 0.371 and 0.749 for comparison of rhythm adjusted means (M), amplitudes (A) and acrophases (), respectively], girls (P=0.993,0.914 and 0.397), or all children (P=0.859,0.712 and 0.865). Differences are found, however, in circasemidian characteristics as well as in the prominent 8-hr ultradian component documented for the short but not for the standard children. These ultradian components should be taken into consideration in the design and later evaluation of a time-specified treatment of children of short stature.     

Racial Differences in the Human Endogenous Circadian Period

The length of the endogenous period of the human circadian clock (tau) is slightly greater than 24 hours. There are individual differences in tau, which influence the phase angle of entrainment to the light/dark (LD) cycle, and in doing so contribute to morningness-eveningness. We have recently reported that tau measured in subjects living on an ultradian LD cycle averaged 24.2 hours, and is similar to tau measured using different experimental methods. Here we report racial differences in tau. Subjects lived on an ultradian LD cycle (1.5 hours sleep, 2.5 hours wake) for 3 days. Circadian phase assessments were conducted before and after the ultradian days to determine the change in circadian phase, which was attributed to tau. African American subjects had a significantly shorter tau than subjects of other races. We also tested for racial differences in our previous circadian phase advancing and phase delaying studies. In the phase advancing study, subjects underwent 4 days of a gradually advancing sleep schedule combined with a bright light pulse upon awakening each morning. In the phase delaying study, subjects underwent 4 days of a gradually delaying sleep schedule combined with evening light pulses before bedtime. African American subjects had larger phase advances and smaller phase delays, relative to Caucasian subjects. The racial differences in tau and circadian phase shifting have important implications for understanding normal phase differences between individuals, for developing solutions to the problems of jet lag and shift work, and for the diagnosis and treatment of circadian rhythm based sleep disorders such as advanced and delayed sleep phase disorder.