出自:河南科技大学大学英语3(专升本)
How much sleep does a person need? 31 the physiological bases of the need for sleep remain conjectural, 32 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 33 from this evidence is 34 there is great variability among individuals in total sleep time. For adults, 35 between six and nine hours of sleep 36 a nightly average is not unusual, and 7.5 hours probably best 37 the norm. Such norms, of course, inevitably vary with the criteria of sleep employed. The most 38 and reliable figures on sleep time, including those 39 here, come from studies in sleep laboratories, where EEG criteria are employed.
_40___ consistently has been associated with the varying amount, quality, and 41 of electro-physi-logically defined sleep. The new born infant may spend an average of about 16 hours of each 24-hour period in sleep, 42 the sleep time 43 sharply; 44 two years of age, it may 45 from 9 to 12 hours. 46 to approximately six hours have been observed among the elderly.
_47___ will be discussed below, EEG sleep studies have indicated that sleep can be considered to 48 of several different stages. Developmental changes in the relative proportion of sleep time in these sleep time 49 in these sleep stages are as 50 as age-related changes in total sleep time.