Changes in cardiovascular function during the sleep onset period in young adults

Blood pressure (BP) and heart rate (HR) are influenced by the sleep-wake cycle, with relatively abrupt falls occurring in association with sleep onset (SO). However, the pattern and rate of fall in BP and HR during SO and the processes that contribute to the fall in these variables have not been fully identified. Continuous BP and HR recordings were collected beginning 1 h before lights out (LO) until the end of the first non-rapid eye movement sleep period in 21 young, healthy participants maintained in a supine position. Five consecutive phases were defined: 1) the 30 min of wakefulness before LO; 2) LO to stage 1 sleep; 3) stage 1 to stage 2 sleep; 4) stage 2 sleep to the last microarousal before stable sleep; and 5) the first 30 min of undisturbed stable sleep. The data were analyzed on a beat-by-beat basis and reported as 2-min periods for phases 1 and 5 and 10% epochs for phases 2, 3, and 4 (as participants had variable time periods in these phases). The level of baroreflex (BR) activity was assessed by the sequence technique and an autoregressive multivariate model. Furthermore, during phases 3 and 4, the BP and HR responses to arousal from sleep were determined. There were substantial falls in BP and HR after LO before the initial onset of θ-activity (phase 3) and again after the onset of stable sleep after the cessation of spontaneous arousals. During phases 3 and 4 when there were repeated arousals from sleep, the fall in both variables was retarded. Furthermore, both the rate and magnitude of the fall in BP were negatively associated with the number of arousals during phases 3 and 4. There was a small increase in the sensitivity of the BR and indirect evidence of a substantial fall in its set point.