Many people feel a mid-afternoon slump in mood and alertness, especially after a poor night of sleep. Many believe that this slump is caused by eating a heavy lunch. However, this usually occurs because we are meant to have a mid-afternoon nap.
Several lines of evidence, including the universal tendency of toddlers and the elderly to nap in the afternoon, as well as the afternoon nap of siesta cultures, have led sleep researchers to the same conclusion: nature intended that we take a nap in the middle of the day. This biological readiness to sleep in the mid-afternoon coincides with a slight drop in body temperature and occurs regardless of whether we eat lunch or not. It is present, even in those who are well rested.
Sleep researchers have also discovered that the afternoon dip in mood and alertness is associated with poorer performance, particularly after a night of sleep loss, and a simultaneous increase in sleep-related accidents. In fact, deaths from all causes show a secondary peak in the afternoon after a nocturnal peak, presumably from sleep-related accidents.
If you don’t snooze, you lose
Other evidence for the biological propensity for a mid-afternoon nap include: drowsiness increases in the mid-afternoon; the afternoon nap is the last nap to be given up by children; older adults revert to afternoon naps; and, adult naps are almost always taken in the afternoon.
A midday nap is an integral part of the daily routine of many cultures, particularly those near the equator. This suggests that napping may have been part of an evolutionary mechanism to get us out of the hot midday sun. However, because the urge for a nap is appreciably weaker than the need to sleep at night, it can be suppressed (or masked by caffeine) but at the cost of increased drowsiness and reduced mood and performance. Also, because naps conflict with work schedules, they are becoming less common in industrialized societies (with the exception of college students and the elderly, who have more regular opportunities to nap). Unfortunately, this decline in napping may be causing poorer afternoon alertness and performance.
