National Geographic : 1987 Dec
SHE MASK OF SLEEP covers a complex sys tem of physiological nges dictated by the ebb I flow of REM and non SM sleep. The synopsis pre ited here is idealized, and all sleep specialists would tee with all details. We enter slumber in stages ined by telltale changes in brain waves, indicated by green stepped lines at left. e compact brain waves of rtness gradually lengthen as drift into non-REM sleep. stage four, the brain waves libit their maximum ampli le, suggesting that this may the most restful sleep. Stage four usually terminates tha posture shift, according some studies. The brain ves now reverse their course the sleeper heads for the ;t REM period of the night. Mon-REM sleep is punctu d by increasingly longer :M periods within 90 nute cycles throughout the ;ht. Stage four sleep is lim I to two or three episodes. Sleepers awakened from M sleep often report vivid, arre dreams, indicated here red and yellow dots. Those 'akened from non-REM ep are more likely to report )ughtlike mentation, indi .ed by muted dots, though ual extravaganzas can occur non-REM periods. Slow, rolling eye move :nts characterize the entry o sleep. In contrast, REM iods are defined by darting )vements whose function is t yet understood. According to one theory, called REM-on neurons in Brain stem generate REM ep. Another theory points to irons in the basal forebrain one of the generators of n-REM sleep.