The time just before you fall asleep is when beneficial cardiovascular changes take place, according to a new study published The American Physiological Society.
The study examined the changes in cardiovascular function of daytime sleep in healthy individuals, comparing napping with other daytime activities such as standing and lying down without going to sleep.
The researchers from the Liverpool John Moores University in Liverpool, United Kingdom, tested nine healthy volunteers (eight men, one woman) who did not routinely take afternoon naps. The volunteers attended the university laboratory on three separate afternoons after sleeping four hours the night before. The volunteers wore equipment that checked blood pressure, heart rate, and forearm cutaneous vascular conductance.
The sessions were broken up into three afternoons: the volunteers spent an hour resting, lying face-up in bed; the volunteer spent an hour relaxed, but standing; and the volunteers was allowed an hour to sleep, lying face-up. During the sleep stage, researchers measured the volunteers’ different stages of sleep.
When the volunteer was allowed to fall asleep, the stage was delineated into three phases:
- Phase 1: A five-minute period of relaxed wakefulness before lights were turned off (volunteers had been lying down for a minimum of 15 minutes before this)
- Phase 2: The period between “lights out” and the onset of Stage 1 sleep (loss of some conscious awareness of the external environment)
- Phase 3: The period between the Stage 1 and the onset of Stage 2 sleep (conscious awareness of the external environment disappears)
The researchers found a significant drop in blood pressure during the sleep trial, but not during the resting or standing trials. The drop in blood pressure occurred mostly after lights out, right before the volunteer fell asleep.
The reduction in blood pressure may be one explanation for the lower cardiovascular mortality that some studies have found among people who habitually take siestas. On the other hand, some studies of nocturnal sleep have shown that blood pressure rises when people awake and more cardiac deaths occur in the morning.
The research team will pursue further studies about blood pressure during the waking portion of an afternoon nap to see if the period poses an increased danger of coronary mortality.