Fig. 3.

Influence of the endogenous circadian timing system on cardiovascular variables at rest and in response to standardized bicycle exercise. Participants undertook an 11-day (including 12 times 20-h “days”) forced desynchrony protocol in which they undertook 15 min of exercise (60% of maximum heart rate) at the same time into each wake period. Data are expressed relative to each participant’s resting value averaged across the whole forced desynchrony protocol (left axes) and in absolute values (right axes) and are plotted according to circadian phase that is separated into six 60°-bins that each equate to ~4 h. Black lines and closed circles indicate resting values, whereas gray lines with open circles represent data obtained during standardized exercise; error bars represent the standard error of the mean. Gray bars indicate the group average habitual clock time for sleep in the 2 weeks prior to admission to the laboratory. The vertical dotted line represents the timing of the group average core body temperature minimum and curly brackets signify the most vulnerable period for adverse cardiovascular events according to epidemiological research (6 AM to noon). Probability data were obtained via cosinor analysis. A statistically significant (P<0.05) second harmonic of a circadian rhythm is indicated by f2 appearing after the probability value. The arrow in the upper right plot is an example of the reactivity of systolic blood pressure to exercise at particular circadian phase. Reproduced with permission from Scheer et al. (2010).