The siesta habit is associated with a 37% reduction in coronary mortality, possibly because of reduced cardiovascular stress associated with daytime sleep. Whether the most important behavior is the daytime nap itself, a supine posture, or the expectancy of a nap is unknown. We present the first detailed description on healthy individuals of the acute changes in cardiovascular function during defined phases of the daytime sleep-onset period. These responses were compared with lying awake and standing. Following a night of restricted (4 h) sleep, nine healthy participants (aged 34 +/- 5 yr) were allowed to sleep at 1400 for up to 1 h. Polysomnography was used to calculate three phases of daytime sleep onset: phase 1, a baseline period of relaxed wakefulness before lights out; phase 2, the period between lights out and onset of stage 1 sleep; and phase 3, the period between onsets of stages 1 and 2 sleep. Differences (means +/- SD) in blood pressure, heart rate, and forearm cutaneous vascular conductance (CVC) between phases were analyzed. During the 9.7 +/- 13.8 min of phase 2, systolic and diastolic blood pressure was 4.7 +/- 4.5 and 3.6 +/- 2.8 mmHg lower than baseline, whereas CVC was 9.5 +/- 4.3% higher than baseline (P < 0.05). Subsequent changes in cardiovascular function during the sleep itself were trivial (P > 0.05). The above changes were not observed when subjects stood or laid supine in relaxed wakefulness for 1 h (P > 0.05). Our findings suggest that the period between lights out and sleep onset is associated with the largest acute reduction in blood pressure during one afternoon siesta.