Relationships between REM and NREM in the NREM-REM sleep cycle: a review on competing concepts

Sixty-five years after the discovery of rapid eye movement (REM) sleep, the reasons why we sleep and why we need two states of sleep are still largely unclear. Moreover, the functional relationship between the two types of sleep remains the matter of much conjecture. Several questions come to mind. How does sleep regulation in monophasic and polyphasic animals compare? What are the circadian and homeostatic influences on both states? Are non-rapid eye movement (NREM) and REM states dependent on each other, or are they regulated independently? What about long-term and short-term regulation? In addition, what determines the number and duration of cycles per night? What roles are played by temperature and energy allocation? The evidence collected over the years regarding these questions is summarized here, trying to address each issue. •This is a broad perspective, historical review of the relationships between REM and NREM sleep.•Deep insights into of the most relevant and competing concepts.•Features a new “asymmetrical” hypothesis on the distal end of ultradian cycles.•REM sleep potentially viewed as an evolutionary remnant of ectotherms sleep.