Knowledge about sleep comes primarily from research on mammalian species, whose daily sleep quotas range from 4 to 19 hours, with rapid eye movement (REM) sleep occupying 10% to 50% of this time. Findings of REM sleep or elements of it in monotremes have filled a gap in its evolutionary history. To some, this suggests that REM was inherited from reptiles, although the absence of REM in living reptiles casts doubt on this view. The function of sleep remains controversial. 1 On one hand,

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... ve theories hold that brain processes during sleep sustain waking behavior (e.g., visual function, learning). On the other hand, the negative correlation of sleep quotas with body size across species suggests that sleep is a state of enforced rest most urgent in species with low energy reserves. Because most of the variance in sleep quotas remains unaccounted for statistically, supplementary theories are in order. There are strikingly strong correlations of REM sleep quotas with degree of maturity at birth-that is, altricial species, born with a low percentage of adult brain weight after a short gestation period, have higher REM sleep quotas, whereas precocial species have lower quotas. Given other fetal characteristics of altricial species (e.g., lapse of thermoregulation), REM sleep may be a carryover from fetal life.