Polygraphic activity was recorded and behavioral observations made during spontaneous sleep and wakefulness in six tree shrews (Tupaia glis). Four distinct states could be distinguished: (1) Wakefulness; characterized by behavioral activity or a standing posture when still, conjugate and nonconjugate eye movements, a low voltage high frequency cortical electroencephalogram (EEG), and a high tonic electromyogram (EMG). (2) Light slow wave sleep (LSWS); characterized by behavioral inactivity, absence of eye movements, 7–8-cps spindles with a slower frequency higher amplitude cortical EEG, and reduced EMG activity. (3) Deep slow wave sleep (DSWS); characterized by behavioral inactivity, a curled-up sleeping posture, quiescent eyes, 7–8-cps spindles with high voltage slow EEG activity, and greatly reduced EMG activity often reaching isoelectric levels. (4) Rapid eye movement (REM) sleep; characterized by a cortical EEG similar to that of wakefulness, except for the presence of phasic potentials (which were not temporally linked with eye movements) and a unique dramatic increase in tonic and phasic EMG activity.Sleep occupied 66% of the 24-hour period, of which 17% was spent in REM sleep, 44% in LSWS, and 39% in DSWS. REM episodes had a mean duration of 3.8 min and recurred during periods of continuous sleep with a mean period of 13.9 min.An increase in tonic EMG activity during REM sleep in the tree shrew contrasts with the decrease in EMG activity observed in all other mammals and lends support to the hypothesis that the evolution of REM sleep might be linked with that of binocularly coordinated eye movement, since the tree shrew has a paucity of conjugate eye movements.