The Circadian Rhythm, also sometimes referred to as body clock, is a 24-hour rhythm aligned to the local sunrise and sunset times primarily by light exposure1 It is also set by breakfast2–5 and other factors.
The Circadian Rhythm consists of hormonal balances that are altered through the course of the day to optimize processes in the body. These include the regulation of melatonin6, which is important for SWS and it also affects the timing of the most effective REM sleep.
Melatonin levels rise at dusk, signalling that sleep should be initiated. The high melatonin levels also lead to the first two sleep cycles containing most of the night’s SWS, and less REM compared to later sleep cycles. The percentage of sleep spent in REM is highest generally between 06:00 and 09:007.
External cues, like light between the frequencies 400 nm and 530 nm8 (blue and green light; from the sun or artificial light sources), food, temperature9 and exercise10 can influence your circadian rhythm. Nowadays, most people have their circadian rhythm shifted forward by evening screentime and artificial light exposure. This makes it difficult for the proper amount of melatonin to build, and can lead to sleep issues. These effects can be mostly negated by filtering the light, and/or restricting the use of electronics within the two hours before bed. Moving the circadian on purpose to sleep later is also possible, but should be done with care. Your circadian is an important and complex hormonal regulatory system. For more info on the effects of not filtering light and moving your circadian rhythm, visit “Lifestyle considerations”.
The circadian rhythm and sleep-wake homeostatic process together are two major processes regulating sleep and alertness. Whereas circadian is set to a generally daily cycle, homeostatic refers to sleep pressure that builds over time, until it is reduced by sleep. REM sleep and non-REM sleep have separate homeostatic pressures. The purpose of any homeostatic process is to maintain equilibrium necessary for optimal day-to-day function of the body.
The higher the homeostatic pressure, i.e. sleep deprivation, the more it will override the circadian pressure at that time. Further, the more sleep deprived you are, the more resistant your brain will be to changing your circadian rhythm11; this is important when attempting a standard schedule shifted late out of the SWS peak. You will want to set your new circadian rhythm first using light and other cues, which can take 1-2 weeks before your homeostatic sleep debt gets too high.
In a healthy, adapted brain, the homeostatic pressure only builds over the course of a day before being reset – or just over a few hours in advanced polyphasic schedules. Polyphasic schedules with naps are designed to relieve homeostatic pressure more frequently than once per day; specifically, naps can relieve REM pressure by containing 10-15 minutes of REM sleep, as well as reducing overall sleep pressure that can be relieved with light sleep. On extreme schedules like uberman, naps are supposed to contain SWS to frequently reduce SWS homeostatic pressure as well.
During polyphasic sleep adaptation, homeostatic pressures can vary rapidly because the brain has not yet adapted to the new sleeping pattern. Instead of returning to equilibrium every day, homeostatic pressures can build up over the course of several weeks. This is why adaptation to the vast majority of schedules is most difficult around week 3. People entering a polyphasic schedule with pre-existing sleep debt (i.e. sleep deprivation) will experience intense homeostatic pressure sooner. This is why polyphasic adaptation is challenging especially when starting already sleep deprived and the difficulty is proportional to the numbers of daily sleep hours reduced. Adapting to new circadian entrainment (e.g. shifted schedules) is also a challenge and it can feel like a jet lag.
The Ultradian rhythm is also sometimes referred to as BRAC. (Basic Rest Activity Cycle)
This rhythm is a lot shorter than the circadian, and repeats itself roughly every 80-120 minutes12–14. BRAC’s are cyclical in manner: the brain starts focused and alert, slowing down for the second half. The last 20-ish minutes the brain is tired and slow. After this, the brain regains focus again, and the process repeats. Sleep cycles are possibly a manifestation of an ultradian rhythm during sleep. BRACs seem to consist of a multitude of different ultradian rhythms14, out of which the one for alertness seems most important for polyphasic sleepers.
Although not the most important factor, it can be a good idea to factor BRAC’s into the planning of your sleep schedule, putting naps at the end of BRAC’s.