单项选择题
| How much sleep does a person need
(31) the physiological bases of the need for sleep remain
conjectural (猜想), rendering conclusive answers to this question impossible, much
evidence has been gathered on how much sleep people do in fact obtain. Perhaps
the most important conclusion to be (32) from this evidence
is (33) there is great variability among individuals in total
sleep time. For adults, (34) between six and nine hours of
sleep as a nightly average is not unusual, and 7.5 hours probably best expresses
the norm. Such norms, of course, forms inevitably vary with the criteria of
sleep employed. The most (35) and reliable figures on sleep
time, including those cited here, come from studies in sleep laboratories, where
EEG criteria are employed. (36) consistently has been associated with the varying amount, quality, and pattern of electrophysiologically defined sleep. The newborn infant may spend an average of about 16 hours of each 24-hour period in sleep, (37) the sleep time drops sharply; by two years of age, it may (38) from nine to 12 hours. Decreases to approximately six hours have been observed among the elderly. (39) will be discussed from below, EEG sleep studies have indicated that sleep can be considered to consist of several different stages. Developmental changes in the relative proportion of sleep time (40) in these sleep stages are as striking as age-related changes in total sleep time. |