The body has many clock-like systems that regulate a myriad of different things. These clocks are found in the esophagus, lungs, liver, pancreas, spleen, thymus, and the skin, as well as the brain! Although they can act independently according to external stimuli such as light, temperature, social interactions, pharmacological manipulation, exercise, and eating/drinking patterns – all healthy systems communicate with each other so that the body can synchronize and make efficient these systems according to what time of day it is, when last your body has performed a function, and external stimuli listed above.
A circadian rhythm is any biological process that displays an endogenous, entrainable oscillation of about 24 hours. When we refer to a circadian rhythm in polyphasic sleep we refer to the rhythm with which the body keeps to detect day and night. The circadian rhythm is usually in sync with local day and night times but doesn’t always have to be (like when jet lag occurs, for example).
An ultradian rhythm is any biological process that displays an endogenous entrainable oscillation of fewer than 24 hours. When we use this term we are referring to the 90-120 minute brain wave frequency cycles that occur when we are both awake and asleep. When we refer to an ‘ultradian’ in polyphasic sleep we refer to the smallest amount of time between the start of your last nap, and the start of your next nap, in a schedule.
The brain has both circadian and ultradian rhythms that control waking and sleeping rhythmic behavior. The brain clock is the suprachiasmatic nucleus (SCN) and is controlled mostly by a light that comes in through the retinas, but there is also internal communication between it and other systems to keep track of the time of day. Sunlight detected by the skin is a day time phenomenon, so the skin can tell the SCN that it is day when UV-B is detected. Eating is an awake process, so the gut can tell the SCN that it is day if food is being processed.
Both the circadian and ultradian rhythm is important because having regular up/down phases is a homeostatic system for regulating all sorts of things, from hormonal secretions, to brain wave frequencies, to blood flow, and brain hemisphere or lobe dominance. This means that without that rhythm, homeostatic functions either work suboptimally or different parts of the body become out of sync with each other.
For example, the gut and brain have two separate ultradian rhythms, but there is integration and communication between them. If there is detectable consistency, such as the presence of day and night, regular eating times, and regular activity, then you will likely have bowel movements when your brain thinks it is morning. If there is no synchronization, both systems are functioning well but have low communication because of changing or confusing circumstances, you will get bowel movements at the same time every ‘gut day’, but there is nothing to say it will be in the morning. Now let’s say there is no regularity in either system and no communication between them, then the cyclic behavior of the gut will become drawn out waiting for signals which could lead to constipation (for example being asleep when a bowel movement is most likely).
Focusing more on the brain now, we can say that there needs the regularity of day and night for the brain to set a circadian rhythm, and without day and night lighting conditions, circadian functions do not complete correctly. For example, the brain will heavily weight sleep toward a certain stage of sleep depending on what time of day it is.
The ultradian rhythm is important for the downregulation of a dominant hemisphere or lobe. When you are concentrating on one task for a long time your brain needs a break from over-depolarization in one area of the brain, so you get a lull in concentration naturally every 90-120mins whilst you are awake. Your brain wave frequencies will rise, then sink. Also, general brain frequency changes in these lulls so that you do not over-stimulate the brain too much high or low-frequency activity and get in a ‘brain wave frequency rut’. When you are asleep (mono) these undulations of brain frequency manifest as sleep stages. every 90-120 minutes (of course sometimes less) your brain wave frequencies will sink, then rise (opposite to when awake).
If you sleep without a rhythm, then those systems become desynchronized with each other, they become less efficient at regulating natural functions. Your circadian rhythm will rotate over the week much more wildly than normal, leading to getting deep sleep in the mornings, then evenings, then nights, then all over the place. Your circadian rhythm will be dysfunctional leading to erratic sleep stage repartitioning, and when you are awake brain wave frequency regulation may lead to unforeseen mental problems. Knowing that ADHD sufferers develop frequency dependencies (parts of their brain will have permanently higher or lower frequency activity than a normal person), as do many other mental disorders, it may be safer to stay regular than to sleep without a rhythm
Circadian Shift is when a person’s circadian rhythm changes over time relative to the actual day time. Sometimes people’s circadian rhythm is not exactly 24 hours, but instead, 24.5h long. Without artificial lighting, a person with this circadian rhythm will have little problem locking in their rhythm and countering this shift because of natural lighting conditions. With artificial lighting and other factors a person’s ‘circadian dawn’ and ‘circadian dusk’ slowly shift forward each day.
Ultradian (or BRAC) Shift is a name for the phenomena where a person’s ultradian rhythm shifts forward as they stay awake longer. If a person with a 2 hour BRAC is following a 4-hour ultradian rhythm (e.g. napping every 4 hours for uberman), if they missed their normal nap and were awake at that time, they would likely next get tired after 2.25 – 2.75 hours, rather than 2 hours. If they missed sleeping then, they would likely get very tired 2.5-3h. Each missed sleep results in a shifting of the ultradian rhythm. As people are awake longer and longer, the shift becomes much wilder and unpredictable until the ultradian rhythm is dominated by a circadian one.
Not everyone’s rhythm is perfect! And rhythmic shift doesn’t always follow the same rules all the time! An ultradian rhythm can shift back and forth over a day in relation to the circadian rhythm, and the circadian rhythm can be flipped on it’s head due to communication with the circadian gut rhythm, or artificial lighting (blue and white light), and of course genetic inconsistencies. Many schedules call for a level of consistency lead by rhythmic sleeping patterns, but after a person has adapted to a schedule to a certain point by teaching their body rhythm and forcing it through the motions, it is often better to listen to the body for when and where naps are going to be placed.
For example, a person may have been following a four-hour ultradian rhythm for 2 weeks straight and is constantly getting tired only 3 hours after a certain sleep each day, that person may be best to listen to their body and sleep when they are tired.
Of course an ‘untrained’ or ‘weak’ rhythm should not be listened to, because the body is not in touch with its own physiological systems, so, for example, a person may have just started a new schedule and is getting tired at the same time each day (at perhaps an unreasonable hour) then the person is to ignore their instinct and continue to teach their body a rhythm for a week or two before listening to their instinct.