Researchers at the Liverpool John Moores University in Liverpool, U.K., have discovered that the time just before one falls asleep in the afternoon is the most beneficial in reducing blood pressure and lessening the risk of cardiovascular diseases. Afternoon naps, or siestas are typically short naps or rest periods of no more than an hour that are taken in the afternoon.
While earlier studies on siestas have found that this practice may slightly increase the risk of heart attack, the new study has shown an inverse relationship between siesta taking and fatal heart attacks.
According to the researchers, change in blood pressure is the key factor linking afternoon naps to cardiovascular function. Some researchers hypothesize that the lower blood pressure reduces strain on the heart and decreases the risk of a fatal heart attack.
The current study provides a detailed description of 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.
For the study, the researchers tested nine healthy volunteers (eight men, one woman) who did not routinely take afternoon naps. The volunteers wore equipment that checked blood pressure, heart rate, and forearm cutaneous vascular conductance (which determines dilation of blood vessels).
During one afternoon session, the volunteer spent an hour resting, lying face-up in bed. During another session, the volunteer spent an hour relaxed, but standing. And in one session, the volunteer was allowed an hour to sleep, lying face-up. During the sleep stage, the researchers measured the volunteer's different stages of sleep.
After analysis, researchers found a significant drop in blood pressure during the sleep trial, but not during the resting or standing trials. Besides, this drop in blood pressure occurred mostly after lights out, just before the volunteer fell asleep.
According to the John Moores team, this fall 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 we awake and that more cardiac deaths occur in the mornings.
Hence, the team will next look at blood pressure during the waking portion of the afternoon nap to see if this period may also pose an increased danger of coronary mortality.
The study entitled, 'Acute Changes in Cardiovascular Function During the Onset Period of Daytime Sleep: Comparison to Lying Awake and Standing,' is published in the online edition of the Journal of Applied Physiology, published by The American Physiological Society.