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, in reality, this occurs because we were 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 and 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 fall asleep in the mid-afternoon coincides with a slight drop in body temperature and occurs regardless of whether we eat lunch. It is present even in good sleepers 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 sleepiness-related accidents. In fact, deaths from all causes show a secondary peak in the afternoon after a nocturnal peak, presumably from sleepiness-related accidents
Other evidence for a biological propensity for a mid-afternoon nap includes: sleepiness increases in the mid-afternoon; the afternoon nap is the last to be given up by children; older adults revert to afternoon naps; and, adult naps are almost always taken in the afternoon;
A mid-day 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 sleepiness 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.
Research on napping suggests that an afternoon nap as short as ten minutes can enhance alertness, mood, and mental performance, especially after a night of poor sleep. Several studies also suggest that polyphasic sleep in the form of 30 minute naps taken regularly (every 4 hours) is the only way to reduce nocturnal sleep below 5-6 hours and still maintain performance. Studies on solo yacht racers indicate that winner's average about five hours of sleep in the form of brief naps throughout the 24 hour day. In one study, subjects were allowed 20 minutes of rest/nap every 6 hours during a 64 hour work period. Results suggested that baseline levels of cognitive functions were maintained.
If you have an opportunity for an afternoon nap, particularly after a poor night of sleep, take one; you will feel more alert and energetic afterwards. Following a mid-afternoon nap, performance may temporarily deteriorate due to grogginess. However, once sleep inertia dissipates (usually 5-20 minutes), mood, energy, and subjective alertness improve beyond baseline; in sleep-deprived individuals, objective alertness and performance also improve. In non-sleep deprived individuals, improvements in performance have also been documented when measured 1.5 to 12 hours after a nap, particularly when naps are scheduled in preparation for all-night work shift. Naps should be limited to 45 minutes and avoided after 4:00 p.m.;otherwise, one may enter deep sleep, which may cause grogginess for a period of time after the nap and reduce the pressure for sleep that night.
Interestingly, there is no evidence that nap benefits are tied to a specific sleep stage. Also, there is evidence that simply resting in the mid-afternoon can improve mood. Sleep itself may not be the crucial factor in the positive effects of afternoon naps on improving mood; what may be important is an afternoon period of relaxation common to both resting and napping.
In summary, napping is normal and beneficial in terms of reducing sleepiness and increasing performance. Optimum human performance appears best served by at least biphasic sleep and perhaps polyphasic sleep.
2010-03-28 19:30 编辑：kuaileyingyu