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.
