Polyphasic sleep theory
The mechanisms of adapting to most polyphasic sleep schedules, especially drastically reducing ones, are largely unknown. Nevertheless, over the decades, people have proposed various hypotheses on how these schedules might work. A few have been developed into full-fledged theories, which are detailed below.
The current most common criteria for adaptation involves subjective reports of restored alertness. However, there have been debates about the underlying mechanism of this process.
Full preservation of SWS and REM
Some believe that in order for an adaptation to be fully successful and sustainable, both SWS and REM must be preserved. These two stages are thus termed vital sleep stages. The basis of this hypothesis lies in the tendency for these two stages to rebound, as opposed to light sleep, which suggest their importance. SWS in particular has been linked to the functioning of the glymphatic system, which is essential to maintaining brain health, and is thus considered indispensable. However, outside of biphasic and non-reducing schedules, there is little EEG evidence backing up this claim.
Possibility of reducing REM
It has also been suggested that the REM baseline from monophasic sleep need not be fully accounted for in a successful adaptation. There has been some EEG evidence of adapted people, especially on Everyman schedules, that this is the case. Another piece of evidence lies in the observation that the percentage of REM stays relatively constant, while the amount may increase, when individuals sleep extra hours due to depression, suggesting that REM requirement might be variable.