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 hou
A. As
B. Despite
C. While
D. Whether
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