How much sleep does a person need?【B1】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【B2】from this evidence is【B3】there is great variability among individuals in total sleep time．For s,【B4】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【B5】and reliable figures on sleep time, including those cited here, come from studies in sleep laboratories, where EEG criteria are employed.
【B6】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,【B7】the sleep time drops sharply; by two years of age, it may【B8】from nine to 12 hours．Decreases to approximately six hours have been observed among the elderly.
【B9】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【B10】in these sleep stages are as striking as age-related changes in total sleep time.
【B1】