Chronobiological considerations for exercise and heart disease

Time of day, but not sleep restriction, affects markers of hemostasis following heavy exercise

We sought to determine the effects of sleep restriction on markers of hemostasis the morning after an exercise session. Seven subjects performed evening exercise followed by an exercise session the next morning, both with and without sleep restriction. Evening exercise included a 20-min submaximal cycling trial (10 min at 50% maximal power (W_max), 10 min at 60% W_max), a 3-km cycling time trial, 60 min of cycling intervals, and 3 sets of leg press. Subsequent morning exercise was the same, excluding intervals and leg press. Blood samples were collected at rest and following the 20-min submaximal trial for factor VIII antigen, tissue plasminogen activator (tPA) activity, and plasminogen activator inhibitor-1 (PAI-1) activity. Sleep restriction had no effect on the variables. Factor VIII antigen was higher and tPA activity lower in the morning versus evening, respectively (P < 0.05). There were larger (P < 0.05) exercise responses for tPA activity in the evening (pre-exercise = 0.32 ± 0.14, postexercise = 1.89 ± 0.60 AU/mL) versus morning (pre-exercise = 0.27 ± 0.13 AU/mL, postexercise = 0.69 ± 0.18 AU/mL). PAI-1 exhibited lower (P < 0.05) responses in the evening (pre-exercise = 0.78 ± 0.26 AU/mL, postexercise = 0.69 ± 0.29 AU/mL) versus morning (pre-exercise = 7.06 ± 2.66, postexercise = 5.40 ± 2.31 AU/mL). Although a prothrombotic environment was observed the morning following an evening exercise session, it was not exacerbated by sleep restriction.

Time-of-day effect on cardiac responses to progressive exercise

This study was designed to examine time-of-day effects on markers of cardiac functional capacity during a standard progressive cycle exercise test. Fourteen healthy, untrained young males (mean ± SD: 17.9 ± 0.7 yrs of age) performed identical maximal cycle tests in the morning (08:00-11:00 h) and late afternoon (16:00-19:00 h) in random order. Cardiac variables were measured at rest, submaximal exercise, and maximal exercise by standard echocardiographic techniques. No differences in morning and afternoon testing values at rest or during exercise were observed for oxygen uptake, heart rate, cardiac output, or markers of systolic and diastolic myocardial function. Values at peak exercise for Vo(2) at morning and afternoon testing were 3.20 ± 0.49 and 3.24 ± 0.55 L min(-1), respectively, for heart rate 190 ± 11 and 188 ± 15 bpm, and for cardiac output 19.5 ± 2.8 and 19.8 ± 3.5 L min(-1). Coefficients of variation for morning and afternoon values for these variables were similar to those previously published for test-retest reproducibility. This study failed to demonstrate evidence for significant time-of-day variation in Vo(2)max or cardiac function during standard progressive exercise testing in adolescent males.

Circadian variation in the circulatory responses to exercise: relevance to the morning peaks in strokes and cardiac events

Sudden cardiac and cerebral events are most common in the morning. A fundamental question is whether these events are triggered by the increase in physical activity after waking, and/or a result of circadian variation in the responses of circulatory function to exercise. Although signaling pathways from the master circadian clock in the suprachiasmatic nuclei to sites of circulatory control are not yet understood, it is known that cerebral blood flow, autoregulation and cerebrovascular reactivity to changes in CO(2) are impaired in the morning and, therefore, could explain the increased risk of cerebrovascular events. Blood pressure (BP) and the rate pressure product (RPP) show marked 'morning surges' when people are studied in free-living conditions, making the rupture of a fragile atherosclerotic plaque and sudden cardiac event more likely. Since cerebral autoregulation is reduced in the morning, this surge in BP may also exacerbate the risk of hemorrhagic and ischemic strokes in the presence of other acute and chronic risk factors. Increased sympathetic activity, decreased endothelial function, and increased platelet aggregability could also be important in explaining the morning peak in cardiac and cerebral events but how these factors respond to exercise at different times of day is unclear. Evidence is emerging that the exercise-related responses of BP and RPP are increased in the morning when prior sleep is controlled. We recommend that such 'semi-constant routine' protocols are employed to examine the relative influence of the body clock and exogenous factors on the 24-h variation in other circulatory factors.

24-hour variation in the reactivity of rate-pressure-product to everyday physical activity in patients attending a hypertension clinic

The exercise-related response of the rate-pressure-product (RPP) is a prognostic marker of autonomic imbalance, cardiovascular mortality, and silent myocardial ischemia in hypertension. In view of the well-known 24 h variation in out-of-hospital sudden cardiac events, our aim was to investigate whether the reactivity of RPP to everyday physical activities varies over the 24 h. Ambulatory measurements of systolic blood pressure (BP) and heart rate were recorded every 20 min for 24 h in 440 diurnally active patients attending a hypertension clinic. Wrist activity counts were summed over the 15 min that preceded a BP measurement. An RPP reactivity index was derived for each of twelve 2 h data bins by regressing the change in RPP against the change in logged activity counts. The RPP showed 24 h variation (p < 0.0005), with a peak of 11,004 (95% CI = 10,757 to 11,250) beat . min(-1) . mmHg occurring at 10:00 h (2 h after mean wake-time). The overall 24 h mean of RPP reactivity was 477 beat . min(-1) . mmHg . logged activity counts(-1) (95% CI = 426 to 529). The largest increase in RPP reactivity occurred within the first 2 h after waking (p < 0.0005). There were no subsequent significant differences in RPP reactivity up to 14 h after waking. The lowest RPP reactivity was found 18-20 h after waking, with a peak-to-trough variation of 593 beat . min(-1) . mmHg . logged activity counts(-1) (95% CI = 394 to 791, p < 0.0005). Although this variation was not moderated by BP status, age, or sex, less variability in RPP reactivity was found for the medicated individuals during the waking hours. These data suggest that under conditions of normal living, the reactivity of RPP to a given change in physical activity increases markedly during the first 2 h after waking from nocturnal sleep, the time when out-of-hospital sudden cardiac events are also most common. Therefore, these data add weight to the notion that reactivity of RPP to physical activity could be a prognostic marker of autonomic imbalance and cardiovascular mortality, although more research is needed to assess the specific prognostic value of 24 h ambulatory measurements of RPP and physical activity.