ଭାଗିଦାରୀ ଧ୍ୱନି

Perceptual disengagement probably serves the function of protecting sleep, so some authors do not count it as part of the definition of sleep itself. But as sleep would be impossible without it, it seems essential to its definition. Nevertheless, many animals (including humans) use the intermediate state of drowsiness to derive some benefits of sleep without total perceptual disengagement.