Long-term measurement of heart rate in chicken eggs

Taking advantage of acoustocardiogram (ACG), we measured the heart rate (HR) of chick embryos continuously from day 12 until hatching and then investigated the development of HR irregularities (HRI), HR variability (HRV), and the existence of a circadian rhythm in mean HR (MHR). HRI comprised transient bradycardia and tachycardia, which first developed on day 14 and 16 in most embryos, respectively. Transient bradycardia increased in frequency and magnitude with embryonic development and occurred over periods of up to 30 min in some embryos. MHR was maximal on around days 14-15 and thereafter decreased to about 250-260 bpm on days 16-18. Baseline HRV, which is an oscillation of the MHR baseline, occurred as HR decreased from days 15-16 and became predominant on days 17-18. The magnitude of the baseline oscillations reached up to 50 bpm in some embryos and the period ranged between about 40-90 min (ultradian rhythm). A circadian rhythm of MHR was not found in late chick embryos. On days 18-19, embryonic activities were augmented and then breathing movements began to occur, disturbing ACG signals and thus making it difficult to measure the HR. Instead, the development of breathing activities was recorded. Breathing frequency was irregular at first and then increased to a maximum of about 1.5 Hz prior to hatching.     

The circadian rhythm of selected parameters of heart rate variability

At present, two main circadian oscillators are known, responsible for the rhythm of body temperature (BT) and body activity. Their independence has been demonstrated by the dissociation of these two rhythms in people during long-term isolation. In order to ascertain the circadian rhythm (CR) of heart rate variability (HRV), the ECG was recorded in 24 healthy awake men every two hours in the sitting position, from Friday 5 p.m. to Monday 6 a.m., who were maintained on a standard regime. One hundred consecutive RR intervals in every ECG were measured and from these 11 selected indicators of HRV were computed. Chronograms from the means of BT, respiratory rate, and electrical skin resistance showed pronounced CR with acrophases at 6 to 8 p.m. "Frequency" parameters of HRV, especially the frequency of reversal points, behaved similarly. CR in the remaining 7 "amplitude" parameters was also detected in individual persons, but their acrophases were different, and averaged chronograms mostly exhibited a flat course. The study has shown that there are at least two circadian components of HRV: the first phase has the CR synchronized with BT and is interindividually more homogeneous; the second phase is synchronized with body activity rhythm and is interindividually heterogeneous. On this basis, three equal subgroups of subjects arose, tentatively called afternoon, night, and forenoon types, respectively, in accordance with information about their preference for working and sleeping.     

Circadian and ultradian rhythms in heart rate variability.

AIM: Heart rate variability (HRV) patterns reflect the changing effect of sympathetic and parasympathetic modulation of the autonomic nervous system. While overall and circadian heart rate (HR) and HRV are well characterized by traditional measures, there is currently no method to measure ultradian cycles of HR and HRV. MATERIALS AND METHODS: HR/HRV for each 2-min interval was calculated using normal-to-normal interbeat intervals from overnight polysomnographic ECGs in 113 subjects, aged 58+/-10 years (65 male, 48 female). HR, SDNN2, high-frequency power (HF) and the LF (low-frequency power)/HF ratio were plotted. A curve-fitting algorithm, developed in MatLab, identified cyclic patterns of HR/HRV and extracted parameters to characterize them. Results were compared for older vs. younger patients, males vs. females, with vs. without severe sleep apnea, and for the upper and lower half of sleep efficiency. RESULTS: Ultradian patterns for different HR/HRV indices had variable correspondences with each other and none could be considered surrogates. Differences were seen for all comparison groups, but no one marker was consistently different across comparisons. CONCLUSION: Each HR/HRV parameter has its own rhythm, and the correspondence between these rhythms varies greatly across subjects. Quantification of ultradian patterns of HRV is feasible and could provide new insights into autonomic physiology.     

Periodic fluctuations of the blood pressure and heart contraction rate in premature newborn infants

To study the postnatal development of circadian rhythm of the blood pressure and the heart rate these parameters were monitored automatically during 24-72 hours at 5h intervals. Fifty infants were investigated at the age 1, 2, 3, and 4 weeks. The results were compared with the cosine curves of different periods (1-48 h) by the IBM PC XT. The fluctuations with all mentioned periods including circadian could be determined in each infant, dominating period being of any duration. In traditional approach the expressiveness of periodical fluctuations is evaluated by the amplitude of cosine curve. Application of the criterion has shown that only amplitude values exceeding 8 mm Hg should be taken into account. Hence, the analysis of BP and HR time series has demonstrated that about 45-50% of the infants have noncircadian rhythms, 20-20% have no significant periodical fluctuations and only 25-30% have circadian rhythm.     

Effect of endurance exercise training on heart rate onset and heart rate recovery responses to submaximal exercise in animals susceptible to ventricular fibrillation.

Both a large heart rate (HR) increase at exercise onset and a slow heart rate (HR) recovery following the termination of exercise have been linked to an increased risk for ventricular fibrillation (VF) in patients with coronary artery disease. Endurance exercise training can alter cardiac autonomic regulation. Therefore, it is possible that this intervention could restore a more normal HR regulation in high-risk individuals. To test this hypothesis, HR and HR variability (HRV, 0.24- to 1.04-Hz frequency component; an index of cardiac vagal activity) responses to submaximal exercise were measured 30, 60, and 120 s after exercise onset and 30, 60, and 120 s following the termination of exercise in dogs with healed myocardial infarctions known to be susceptible (n = 19) to VF (induced by a 2-min coronary occlusion during the last minute of a submaximal exercise test). These studies were then repeated after either a 10-wk exercise program (treadmill running, n = 10) or an equivalent sedentary period (n = 9). After 10 wk, the response to exercise was not altered in the sedentary animals. In contrast, endurance exercise increased indexes of cardiac vagal activity such that HR at exercise onset was reduced (30 s after exercise onset: HR pretraining 179 +/- 8.4 vs. posttraining 151.4 +/- 6.6 beats/min; HRV pretraining 4.0 +/- 0.4 vs. posttraining 5.8 +/- 0.4 ln ms(2)), whereas HR recovery 30 s after the termination of exercise increased (HR pretraining 186 +/- 7.8 vs. posttraining 159.4 +/- 7.7 beats/min; HRV pretraining 2.4 +/- 0.3 vs. posttraining 4.0 +/- 0.6 ln ms(2)). Thus endurance exercise training restored a more normal HR regulation in dogs susceptible to VF.     

Different times of day do not change heart rate variability recovery after light exercise in sedentary subjects: 24 hours Holter monitoring

Incidence of cardiovascular events follows a circadian rhythm with peak occurrence during morning. Disturbance of autonomic control caused by exercise had raised the question of the safety in morning exercise and its recovery. Furthermore, we sought to investigate whether light aerobic exercise performed at night would increase HR and decrease HRV during sleep. Therefore, the aim of this study was to test the hypothesis that morning exercise would delay HR and HRV recovery after light aerobic exercise, additionally, we tested the impact of late night light aerobic exercise on HR and HRV during sleep in sedentary subjects. Nine sedentary healthy men (age 24 ± 3 yr; height 180 ± 5 cm; weight 79 ± 8 kg; fat 12 ± 3%; mean±SD) performed 35 min of cycling exercise, at an intensity of first anaerobic threshold, at three times of day (7 a.m., 2 p.m. and 11 p.m.). R-R intervals were recorded during exercise and during short-time (60 min) and long-time recovery (24 hours) after cycling exercise. Exercise evoked increase in HR and decrease in HRV, and different times of day did not change the magnitude (p < 0.05 for time). Morning exercise did not delay exercise recovery, HR was similar to rest after 15 minutes recovery and HRV was similar to rest after 30 minutes recovery at morning, afternoon, and night. Low frequency power (LF) in normalized unites (n.u.) decreased during recovery when compared to exercise, but was still above resting values after 60 minutes of recovery. High frequency power (HF-n.u.) increased after exercise cessation (p < 0.05 for time) and was still below resting values after 60 minutes of recovery. The LF/HF ratio decreased after exercise cessation (p < 0.05 for time), but was still different to baseline levels after 60 minutes of recovery. In conclusion, morning exercise did not delay HR and HRV recovery after light aerobic cycling exercise in sedentary subjects. Additionally, exercise performed in the night did change autonomic control during the sleep. So, it seems that sedentary subjects can engage physical activity at any time of day without higher risk.     

Disrupted circadian rhythms of body temperature,heart rate and fasting blood glucose in prediabetes and type 2 diabetes mellitus

We report a progressive disruption of 24-h rhythms in fasting blood glucose (FBG), body temperature (BT) and heart rate (HR) associated with metabolic dysfunction and the development of prediabetes (PD) and type 2 diabetes mellitus (T2DM) in overweight middle-aged (40–69 years old) humans. Increasing BT and HR mean values and declining 24-h BT and HR amplitudes accompany adverse changes in metabolic state. Increased nocturnal BT and a phase delay of the 24-h BT rhythm, deviant 24-h HR profile and a phase advance of the 24-h HR and FBG rhythms are early signs of the PD metabolic state. In T2DM, the 24-h FBG rhythm is no longer detectable, and the 24-h amplitudes of BT and HR are greatly diminished. In addition, lepton and creatinine values were lowered in T2DM. Moreover, positive correlations between FBG and body mass index, BMI, and negative correlations between the 24-h amplitude of FBG and BMI indicate that overweight is an additional factor causing disruption of the circadian rhythms. Further studies on circadian disruption as a consequence of metabolic dysfunction are necessary. The quantitative analysis of changing circadian BT and HR rhythms may provide prognostic markers of T2DM and therapeutic targets for its prevention and correction.     

Circadian rhythm in QT interval is preserved in mice deficient of potassium channel interacting protein 2

Potassium Channel Interacting Protein 2 (KChIP2) is suggested to be responsible for the circadian rhythm in repolarization duration, ventricular arrhythmias, and sudden cardiac death. We investigated the hypothesis that there is no circadian rhythm in QT interval in the absence of KChIP2. Implanted telemetric devices recorded electrocardiogram continuously for 5 days in conscious wild-type mice (WT, n = 9) and KChIP2^?/? mice (n = 9) in light:dark periods and in complete darkness. QT intervals were determined from all RR intervals and corrected for heart rate (QT₁₀₀ = QT/(RR/100)^1/2). Moreover, QT intervals were determined from complexes within the RR range of mean-RR ± 1% in the individual mouse (QT_mean-RR). We find that RR intervals are 125 ± 5 ms in WT and 123 ± 4 ms in KChIP2^?/? (p = 0.81), and QT intervals are 52 ± 1 and 52 ± 1 ms, respectively(p = 0.89). No ventricular arrhythmias or sudden cardiac deaths were observed. We find similar diurnal (light:dark) and circadian (darkness) rhythms of RR intervals in WT and KChIP2^?/? mice. Circadian rhythms in QT₁₀₀ intervals are present in both groups, but at physiological small amplitudes: 1.6 ± 0.2 and 1.0 ± 0.3 ms in WT and KChIP2^?/?, respectively (p = 0.15). A diurnal rhythm in QT₁₀₀ intervals was only found in WT mice. QT_mean-RR intervals display clear diurnal and circadian rhythms in both WT and KChIP2^?/?. The amplitude of the circadian rhythm in QT_mean-RR is 4.0 ± 0.3 and 3.1 ± 0.5 ms in WT and KChIP2^?/?, respectively (p = 0.16). In conclusion, KChIP2 expression does not appear to underlie the circadian rhythm in repolarization duration.     

Geological cycles

Abstract

Deep body temperature (DBT) and heart rate (HR) circadian rhythms were determined by radiotelemetry in 4 mares kept under controlled light and temperature conditions. Ovulations were determined by rectal palpation of their ovaries. Mean DBT values ranged from 35.85 ± .04 to 37.22 ± .02°C The circadian range of oscillation was extremely low, approximately 0.5° C, with time of maximum temperature occurring midway through the dark period. Mean HR values ranged from 36.4 ± 1.7 to 53.0 ±3.6 beats per min. The circadian range of oscillation was also low, less than 15 beats per min with time of maximum HR occurring approximately at the time of lights off. The HR circacadian rhythm peaked before the DBT circadian rhythm by 3 to 8 hrs. Ovulation did not appear to consistently affect DBT and HR circadian rhythms or their phase relationships.     

Effects of endurance exercise training on heart rate variability and susceptibility to sudden cardiac death: protection is not due to enhanced cardiac vagal regulation.

Low heart rate variability (HRV) is associated with an increased susceptibility to ventricular fibrillation (VF). Exercise training can increase HRV (an index of cardiac vagal regulation) and could, thereby, decrease the risk for VF. To test this hypothesis, a 2-min coronary occlusion was made during the last min of a 18-min submaximal exercise test in dogs with healed myocardial infarctions; 20 had VF (susceptible), and 13 did not (resistant). The dogs then received either a 10-wk exercise program (susceptible, n=9; resistant, n=8) or an equivalent sedentary period (susceptible, n=11; resistant, n=5). HRV was evaluated at rest, during exercise, and during a 2-min occlusion at rest and before and after the 10-wk period. Pretraining, the occlusion provoked significantly (P<0.01) greater increases in HR (susceptible, 54.9+/-8.3 vs. resistant, 25.0+/-6.1 beats/min) and greater reductions in HRV (susceptible, -6.3+/-0.3 vs. resistant, -2.8+/-0.8 ln ms2) in the susceptible dogs compared with the resistant animals. Similar response differences between susceptible and resistant dogs were noted during submaximal exercise. Training significantly reduced the HR and HRV responses to the occlusion (HR, 17.9+/-11.5 beats/min; HRV, -1.2+/-0.8, ln ms2) in the susceptible dogs; similar response reductions were noted during exercise. In contrast, these variables were not altered in the sedentary susceptible dogs. Posttraining, VF could no longer be induced in the susceptible dogs, whereas four sedentary susceptible dogs died during the 10-wk control period, and the remaining seven animals still had VF when tested. Atropine decreased HRV but only induced VF in one of eight trained susceptible dogs. Thus exercise training increased cardiac vagal activity, which was not solely responsible for the training-induced VF protection.     

Resonance of about-weekly human heart rate rhythm with solar activity change

In several human adults, certain solar activity rhythms may influence an about 7-day rhythm in heart rate. When no about-weekly feature was found in the rate of change in sunspot area, a measure of solar activity, the double amplitude of a circadian heart rate rhythm, approximated by the fit of a 7-day cosine curve, was lower, as was heart rate corresponds to about-weekly features in solar activity and/or relates to a sunspot cycle.     

Shortening of atrioventricular delay at increased atrial paced heart rates improves diastolic filling and functional class in patients with biventricular pacing

Background

Use of rate adaptive atrioventricular (AV) delay remains controversial in patients with biventricular (Biv) pacing. We hypothesized that a shortened AV delay would provide optimal diastolic filling by allowing separation of early and late diastolic filling at increased heart rate (HR) in these patients.

Methods

34 patients (75 ± 11 yrs, 24 M, LVEF 34 ± 12%) with Biv and atrial pacing had optimal AV delay determined at baseline HR by Doppler echocardiography. Atrial pacing rate was then increased in 10 bpm increments to a maximum of 90 bpm. At each atrial pacing HR, optimal AV delay was determined by changing AV delay until best E and A wave separation was seen on mitral inflow pulsed wave (PW) Doppler (defined as increased atrial duration from baseline or prior pacemaker setting with minimal atrial truncation). Left ventricular (LV) systolic ejection time and velocity time integral (VTI) at fixed and optimal AV delay was also tested in 13 patients. Rate adaptive AV delay was then programmed according to the optimal AV delay at the highest HR tested and patients were followed for 1 month to assess change in NYHA class and Quality of Life Score as assessed by Minnesota Living with Heart Failure Questionnaire.

Results

81 AV delays were evaluated at different atrial pacing rates. Optimal AV delay decreased as atrial paced HR increased (201 ms at 60 bpm, 187 ms at 70 bpm, 146 ms at 80 bpm and 123 ms at 90 bpm (ANOVA F-statistic = 15, p = 0.0010). Diastolic filling time (P < 0.001 vs. fixed AV delay), mitral inflow VTI (p < 0.05 vs fixed AV delay) and systolic ejection time (p < 0.02 vs. fixed AV delay) improved by 14%, 5% and 4% respectively at optimal versus fixed AV delay at the same HR. NYHA improved from 2.6 ± 0.7 at baseline to 1.7 ± 0.8 (p < 0.01) 1 month post optimization. Physical component of Quality of Life Score improved from 32 ± 17 at baseline to 25 ± 12 (p < 0.05) at follow up.

Conclusions

Increased heart rate by atrial pacing in patients with Biv pacing causes compromise in diastolic filling time which can be improved by AV delay shortening. Aggressive AV delay shortening was required at heart rates in physiologic range to achieve optimal diastolic filling and was associated with an increase in LV ejection time during optimization. Functional class improved at 1 month post optimization using aggressive AV delay shortening algorithm derived from echo-guidance at the time of Biv pacemaker optimization.     

Circadian changes of heart rate in West syndrome

Patterns of circadian and ultradian rhythms in the heart rate (HR) are described in a full-term baby with birth asphyxia and convulsions. A 24h HR recording was carried out at the age of 1, 15, 56, 289, and 295 days; West syndrome diagnosis was made when the patient was 3 months old. The HR showed no circadian rhythm in the follow-up, whereas it is known that the circadian rhythm appears in healthy infants at the age of 1 month and remains thereafter. This observation may be an indirect indicator of the interference of West syndrome with centers of neurological maturity. (Chronobiology International, 17(4), 591-595, 2000)     

Age-Related Circadian Rhythm of DHEAS Plasma Levels in Male Subjects

The aging influences the endocrine temporal structure, including DHEAS which can be considered as a biomarker of aging, since its levels gradually decrease in older subjects. The aim of this work was to observe the circadian rhythms of DHEAS, prolactin, cortisol and body temperature, in healthy elderly male subjects (73.7 ± 2.5 years) compared with healthy young subjects (27.2 ± 6.6 years). The results documented that in our subjects no significant age-related differences in prolactin levels existed. In elderly subjects cortisol levels were weakly enhanced in comparison with young subjects. DHEAS showed a preserved circadian rhythm, but markedly lower rhythm adjusted mean (74.38 ± 10.29 versus 273.63 ± 26.39) (p &lt; 0.001) and decreased amplitude of oscillation (p &lt; 0.001), when expressed as absolute value, in elderly subjects when compared with young subjects. In elderly subjects the DHEAS circadian rhythm modifications could represent an impairment of of the endocrine temporal structure.     

Heart rate circadian rhythm as a biological marker of desynchronization in major depression: a methodological and preliminary report

Heart rate (HR) was continuously monitored during successive 24-hr periods in 19 healthy subjects and 26 major depressed patients (DSM III-R). Recordings were performed after a 2-week wash-out period and the morningness or eveningness typology of each subject was determined. The chronobiological parameters and rhythm percentage (RP) were calculated by the single cosinor method from the smoothed HR curves of each subject. In normal subjects, HR follows a circadian rhythm (RP greater than 65%) with the lowest values at night. Morning type subjects have an earlier peak time (13:30) than evening type subjects (17:30). Major depressive patients were split into two groups; in the first one HR circadian rhythm was still present (RP greater than 63%) with a decrease in amplitude (24%) while in the second group, no circadian rhythm of HR could be detected (RP less than 25%, decrease in amplitude greater than 70%). In the group of patients with a persisting HR circadian rhythm, no veritable phase advance was observed. Our results suggest that circadian HR rhythm, which can be easily studied with non-invasive methods, might represent a chronobiological marker of some depressions. Given the lag that exists between the rhythms of morning type and evening type subjects, our study also stresses the importance of taking into account this behavioural trait in chronobiological studies.     

Human Circadian Time Structure in Subjects of Different Gender and Age

Most human variables exhibit rhythms with an about 24 hour (circadian) period. Each rhythm can be characterized by its acrophase (calculated peak time of the cosine curve best fitting to the data), its amplitude and rhythm adjusted mean (MESOR). The sequential array of the rhythms' acrophases represents the temporal order of the human time structure. In the present work we used circadian rhythms of 24 chemical and 15 hormonal variables extracted from published studies which were done in a defined area of southeastern Europe (Romania). All studies had a comparable experimental design and were analyzed biochemically and statistically in the same laboratory. The acrophases of these rhythms obtained from both genders of different age groups (from the 2nd to the 9th decade of age) were subjected to multiple correlation test, cluster and principal coordinates analyses. The results show that the temporal order is affected both by gender and age, and evaluate the degree of the effect, offer a “chronbiologic fingerprint” for the examined groups and assist in dissecting rhythm variability among populations.     

CARDIOVASCULAR RHYTHMS AND CARDIAC BAROREFLEX SENSITIVITY IN AT1A RECEPTOR GAIN-OF-FUNCTION MUTANT MICE

A mutant mouse expressing a gain-of-function of the AT_1A angiotensin II receptor was engineered to study the consequences of a constitutive activation of this receptor on blood pressure (BP). Cardiovascular rhythms and spontaneous cardiac baroreflex sensitivity (BRS) were evaluated using telemetric BP recordings of five transgenic (AT_1AMUT) and five wild (AT_1AWT) mice. The circadian rhythms were described with the Chronos-Fit program. The gain of the transfer function between systolic BP (SBP) and pulse intervals used to estimate the spontaneous BRS (ms/mmHg) was calculated in the low frequency (0.15–0.60?Hz) band. Transgenic AT_1AMUT exhibited higher BP and heart rate (HR) levels compared to controls (SBP AT_1AMUT 134.6?±?5.9?mmHg vs. AT_1AWT 110.5?±?5.9; p?<?0.05; HR AT_1AMUT 531.0?±?14.9 vs. AT_1AWT 454.8?±?5.4 beats/min; p?=?0.001). Spontaneous BRS was diminished in transgenic mice (AT_1AMUT 1.23?±?0.17?ms/mmHg vs. AT_1AWT 1.91?±?0.18?ms/mmHg; p?<?0.05). Motor activity did not differ between groups. These variables exhibited circadian changes, and the differences between the strains were maintained throughout the cycle. The highest values for BP, HR, and locomotor activity were observed at night. Spontaneous BRS varied in the opposite direction, with the lowest gain estimated when BP and HR were elevated (i.e., at night, when the animals were active). It is likely the BP elevation of the mutant mice results from the amplification of the effects of AngII at different sites. Future studies are necessary to explore whether AT_1A receptor activation at the central nervous system level effectively contributed to the observed differences. (Author correspondence: jean-luc.elghozi@nck.aphp.fr)     

Human Circadian Time Structure in Subjects of Different Gender and Age

Most human variables exhibit rhythms with an about 24 hour (circadian) period. Each rhythm can be characterized by its acrophase (calculated peak time of the cosine curve best fitting to the data), its amplitude and rhythm adjusted mean (MESOR). The sequential array of the rhythms' acrophases represents the temporal order of the human time structure. In the present work we used circadian rhythms of 24 chemical and 15 hormonal variables extracted from published studies which were done in a defined area of southeastern Europe (Romania). All studies had a comparable experimental design and were analyzed biochemically and statistically in the same laboratory. The acrophases of these rhythms obtained from both genders of different age groups (from the 2nd to the 9th decade of age) were subjected to multiple correlation test, cluster and principal coordinates analyses. The results show that the temporal order is affected both by gender and age, and evaluate the degree of the effect, offer a “chronbiologic fingerprint” for the examined groups and assist in dissecting rhythm variability among populations.     

Body temperature rhythm of the tree shrew, Tupaia belangeri.

The circadian rhythm of body temperature of the tree shrew Tupaia belangeri was studied by telemetry. The amplitude of the daily (or circadian) variation was found to be much larger than that of most endotherms (amplitude approximately 5 degrees C) and the bimodal shape of the rhythm differed from the cosine waveform that characterizes the temperature rhythms of most other species. In free-running conditions, as well as in the entrained state, the temperature rhythm remained synchronized to the rhythm of locomotor activity.     

Frequency characteristics of long-term heart rate variability during constant-routine protocol

The effects of such behavioral factors as physical activity, food intake, and circadian rhythm on long-term heart rate variability (HRV) in humans remain poorly understood. We therefore studied their effects on HRV using a constant-routine protocol that included simultaneous core body temperature (CBT) correction. Seven healthy subjects completed the constant-routine and daily-routine protocols, during which HRV and CBT were continuously monitored. During the constant routine, subjects were kept awake for 27 h in a semirecumbent posture with minimal physical activity; small isocaloric meals were provided every 2 h. During the daily routine, subjects carried on their lives normally. Data were analyzed using generic spectral analysis based on a fast Fourier transform; coarse-graining spectral analysis was also used to eliminate periodicity due to the regular meals for raw HRV and for the CBT-corrected HRV without circadian and/or low-frequency ultradian components. The results showed that 1) the power spectra of HRV in the constant routine and daily routine had similar power-law scalings at frequencies above approximately 10(-3.5) Hz, while 2) below that crossover frequency, HRV was smaller in the constant routine than in the daily routine, with the difference becoming significant (P < 0.05) at <10(-4) Hz, 3) coarse-graining spectral analysis eliminated diet-induced peaks in generic spectral analysis-based HRV spectra during the constant routine and emphasized the crossover at approximately 10(-3.5) Hz, and 4) CBT correction did not alter the results. Below a frequency of approximately 10(-3.5) Hz (a period >1 h), HRV is strongly influenced by behavioral factors; above that crossover frequency, HRV is behavior independent, possibly reflecting an intrinsic regulatory system.