Imp: Sleep

Guide to Good Night’s Sleep
p.18People tested to measure how well they
have learned a new task improve their scores after a night’s sleep.
If repeatedly roused from REM sleep, however, the improvements
are lost. On the other hand, if they are awakened an equal
number of times from slow-wave sleep, the improvements in the
scores are unaffected. Such findings may help explain why students
who stay up all night cramming for an examination generally
retain less information than classmates who get some sleep.
Three to five times a night, or about every ninety minutes, a
sleeper enters REM sleep. The first such episode usually lasts for
only a few minutes, but REM time increases progressively over
the course of the night. The final period of REM sleep may last
half an hour. Altogether, REM sleep makes up about 25 percent
of total sleep in young adults. If someone who has been deprived
of REM sleep is left undisturbed for a night, he or she enters this
stage earlier and spends a higher proportion of sleep time in it—
a phenomenon called REM rebound.