REM sleep: a biological and psychological paradox.

Sleep and rest can be satisfactorily explained as adaptive states1,2 whose core function is energy conservation and behavioral regulation. In addition to these functions, certain recuperative processes may be accomplished within sleep. However, the adaptive role of rapid eye movement (REM) sleep remains a complete mystery. The high levels of brain metabolic demand and attenuation of homeostatic regulationmake it difficult to understand how animals benefit from this state. Adding to the mystery, it is well known that drug induced suppression of REMsleep iswithout any striking effect on behavior. Humans taking selective serotonin reuptake inhibitors, serotonin–norepinephrine reuptake inhibitors,monoamine oxidase inhibitors and antidepressants have a massive suppression of REM sleep3 yet they do not go insane and do not have any apparent impairment of memory – indeed may have some small improvements of memory.4 Even long term usage of these drugs, with concomitant reduction or elimination of REM sleep does not generally havedeleterious effects onhealth. Indeed they are often given to regulate blood pressure or improvemood. Similarly, no cognitive or health impairment has been identified in the fewhumanswho have lost REM sleep because of brain injury.5 So, REM sleep truly has earned the name “paradoxical sleep,” first given to it by the French researcher Michel Jouvet because of its waking electroencephalogram (EEG) during behavioral sleep. It should be noted that in contrast to the findings in humans, deprivation of rats of REM or non REM sleep by the disk over water method can be lethal.6 However, this may well be due to stress involved in repeated arousals rather than sleep loss itself. Lethal effects of sleep loss have not been reported in mice, cats or other animal sleep subjects. Soweare leftwith a profoundquestion;whydoall landmammals (but not necessarilymarinemammals1,7) andmany, perhaps all birds have REM sleep8 if it uses so much energy and can apparently be dispensed with without obvious negative consequences? Several recent papers in this journal address aspects of this paradox.9–14