Rapid eye movement (REM) sleep is a unique phenomenon within sleep-wakefulness cycle. It is associated with increased activity in certain group of neurons and decreased activity in certain other group of neurons and dreaming. It is likely to have evolved about 140 million years ago. Although mention of this stage can be traced back to as early as 11 century BC in the Hindu Vedic literature, the Upanishads, it has been defined in its present form in the mid-twentieth century. So far, neurobiology of its genesis, physiology and functional significance are not known satisfactorily and mostly remains hypothetical. Nevertheless, more and more studies have increasingly convinced us to accept that it is an important physiological phenomenon which cannot be ignored as a vestigial phenomenon. Although there are articles where different aspects of REM sleep have been dealt with, a review where the knowledge gathered by REM sleep deprivation studies to understand its significance is lacking. There is a need for such a review because a major portion of the knowledge about various aspects of REM sleep, specially its functional significance, has been acquired mostly from the REM sleep deprivation studies. Hence, in this review the knowledge gathered by REM sleep deprivation studies have been colated along with their importance so that it may be useful and referred to for information as well as while designing future studies.