The siesta habit is associated with a 37% reduction in coronary mortality, possibly due to 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, 9 healthy participants, aged 34±5 years, were allowed to sleep at 14:00 h for up to 1 hour. Polysomnography was used to calculate 3 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 I sleep, and Phase 3, the period between onsets of stages I and II sleep. Mean±SD differences in blood pressure, heart rate and forearm cutaneous vascular conductance (CVC) between phases were analyzed. During the 9.7±4.6 min of phase 2, systolic and diastolic blood pressure was 4.7±4.4 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.