Our declining sleep duration over early human infant development is largely through REM sleep (REM), loss, not of nonREM. It coincides with the infant's increasing locomotion providing for multisensory inputs (‘exploratory wakefulness’ – EW), together facilitating neural restructuring and behavioural adaptations (‘neuroplasticity’). EW also involves curiosity, novelty, navigation, spatial memory, associated emotions, and feeding; all having brain processes particularly active in REM. It is proposed that: 1) REM is a proxy for EW in facilitating neuroplasticity; 2) necessitating REM having a locomotor output, actively inhibited (the atonia); 3) human adults retain many (neotenous) infant characteristics including large amounts of REM towards the end of usual sleep, where REM's qualitative changes indicate reduced sleep pressure, 4) as in infancy, some of our adult REM remains replaceable by EW (without REM rebounds), mostly in this final REM episode whenever EW need prevails. Accordingly, our adult sleep duration is adaptable to habitual shortening via this REM episode substituted by purposeful EW, which could provide extra (day) light exposure for circadian synchrony. Such processes may underlie seasonally shorter (6 h) sleep, eg in hunter-gather people. This flexibility of REM questions the extent of our western ‘chronic sleep debt’. Evidence is provided to counter claims that this absent REM would cause obesity and related disorders. 200w.