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DREAMING SLEEP IN MAN: A REAPPRAISAL J. A. HORNE* In man, rapid eye movement (REM) sleep [1] (dreaming sleep) is probably the most interesting of all the sleep stages, by virtue of its phenomena of dreaming, rapid eye movements, loss of muscle tonus, and penile erections, and much of sleep research has been oriented toward it. Even though REM only occupies about 25 percent of total sleep time in adult man, much of the theory and research into sleep function is oriented toward REM. Nevertheless, REM can be considered as a "fragile" [2] state of sleep, because in man and in other mammals it tends to be reduced and substituted by light sleep and awakenings whenever such adverse sleeping circumstances as stress, extremes in temperature, or unusual environments are present. Furthermore, nondomestic mammal species living naturaUy under threat of prédation have habitually low absolute and relative amounts of REM [2-4], whilst predators tend to have more. This sleep safety factor appears to be independent ofbody size [3]; although it has also been shown that body size itself does influence REM [4]. The fragility and apparent adaptabiUty of REM might suggest that it may not be such a critical type of sleep as is often thought. The aim of the present discussion is to further evaluate this possibility, that despite the intricacies of and the interest paid to REM, its apparent importance may need to be relegated. As will be seen, ofthe remaining human sleep stages, stages 1, 2, 3, and 4 [1], the latter two stages commonly called "slow wave sleep" (SWS) may be of equal or greater importance to man dian is REM. Although it is generally beüeved that REM is oriented toward brain "recovery," the discussion also makes the case that there is as yet little firm evidence in support ofthis beliefand that an argument can be made for REM having an energy conserving role for the body. This discussion concerns itself mostly with man, rather than with mammals in general, because it cannot yet be assumed that sleep serves the same functions for all mammals, as differences in cortical develop- ?Department of Human Sciences, Loughborough University, Loughborough, Leicestershire , LEU 3TU, England.© 1978 by The University of Chicago. 0031-5982/78/2104-0021$01.00 Perspectives in Biology and Medicine ¦ Summer 1978 \ 591 ment, body size and safety [3, 4] have all been shown to affect die quality and quantity of sleep, and underlying sleep mechanisms [2], The Relegation ofREM in Man Although SWS is encompassed by the commonly used term "nonREM ," a reference which reflects a putative importance of REM, SWS generally appears, in the adult, to take precedence over REM. For example, from a simple but logical standpoint it might be expected that the most important type of sleep would be very evident in the initial hours of the normal sleep period. It is not REM which tends to make such an appearance, but SWS, with REM to be found mosdy in the second halfof sleep. Even with experimental manipulation of sleep it is difficult, in the adult, to encourage REM to occur in any quantity at die beginning of the normal sleep period [5]. More evidence ofan apparent SWS priority over REM comes from aU reports of recovery sleep foUowing total sleep deprivation (e.g. [6-8]). SWS shows large increases (rebounds) on the first one or two recovery nights, with a REM rebound not becoming really apparent until the second night and thereafter, when the SWS rebound is seemingly nearer completion. Although there is usually more REM than SWS in a normal night's sleep ofa young adult, the overall extent ofthe rebound for SWS is higher than for REM. AU reports of enforced Umited sleep regimes (e.g. [9-12]) show that subjects, having had time to adapt to these circumstances, tend to devote about as much absolute time to SWS as they normally do, whereas it is REM and the Ughter stages 1 and 2 of sleep which are reduced. For example, a 3-hour regime [9] produced a 90 percent reduction in REM but little change in SWS. The only two studies [13, 14] of naturaUy occurring very...


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