The groggy feeling immediately after awakening from
napping, resulting in impaired alertness and performance,
is known as sleep inertia [16]. Sleep inertia is
caused by the physiological delay associated with
switching of the brain from a state of sleep to one of
full alertness. The effects of sleep inertia usually
dissipate in 30 min. Sleep inertia slows the speed of
cognitive tasks, but has few effects on the accuracy of
task performance [16]. As such, the impact of sleep
inertia is not necessarily regarded as the basis for not
advocating napping. Rather, it is essential to actively
cope with sleep inertia. Since awakening from deep
NREM sleep makes sleep inertia more severe, a nap
should be either shorter than 20 min (to avoid the
likelihood of deep NREM sleep) or approximately
90 min (to allow suf®cient time for one NREM-REM
cycle and waking from REM sleep). It is also important
to learn how to resume activities after waking up from
a nap. We need to pause before returning to work and
avoid tasks that require a quick response or critical
judgments immediately after awakening.