Sleep Cycles: going full circle is important?
History of sleep patterns
Humans are in general monophasic sleepers. This means that our sleep pattern consists of one episode of consolidated sleep and a main episode of wakefulness. Polyphasic sleepers have a pattern of frequent wakefulness and sleep during the whole day.
It is theorized that during the Middle Paleolithic period (70,000-40,000 BCE) the Neanderthals changed their sleep pattern from polyphasic to monophasic. Scientist formed this theory as a result of studying the sleep and waking patterns of nonhuman primates. The sleep patterns of these primates consists mostly of polyphasic patterns, it is assumed that this sleep cycle provides security from predators. This is why scientists believe that the earliest humans had a similar pattern. In ancient times humans tried to influence their sleep patterns with certain treatments like bloodletting and medicine, in Ancient India they used the plant called Rauwolfia Serpentina to treat insomnia.
The first sleep studies on circadian rhythms
One of the first known scientists that studied sleep was Moses Maimonides, a physician from 1180, who claimed that monophasic sleep that lasts one-third of a day is sufficient for humans. Scientists started to wonder if our sleep patterns were determined by the circadian rhythms in our bodies or by external factors like daylight. In 1729 Jean Jacques d’Ortuous de Marian did an experiment on plants to determine if they had circadian rhythms; he chose specimens that open and close their leaves during day and night. He concluded that plants have circadian rhythms that regulated their sleep patterns, even when blocked out from any light.
In 1924, Hans Berger recorder the human brain with an electroencephalogram (EEG) and found that brains have different wave patterns between sleep and wakefulness. In 1952 two researchers, Nathaniel Kleitman and Bruce Richardson, tried to determine if humans have circadian rhythms but they only discovered rapid eye movement (REM) – a normal sleep phase that occurs 4 to 5 times per night, in this phase vivid dreams and lighter sleep are experienced.
In 1958 sleep researcher Jurgen Aschoff tried the same experiment and he concluded that our sleep patterns are indeed dependent on circadian rhythms. In 1968, Allen Rechtschaffen and Anthony Kales published their EEG manual of their R&K criteria to score sleep stages. This manual has four non-REM stages and one REM stage of sleep.
Thanks to the scientists who analyzed the brain with electroencephalogram (EEG) we now know sleep can be categorized in different sleep stages. We also know that during our sleep we can either be in a state called non-rapid eye movement (NREM) sleep, also called “quiet sleep”, or rapid eye movement (REM) sleep, also called “active sleep” or “paradoxical sleep”.
Each sleep stage within any sleep cycle has a certain physiological and neurological function that benefits the health of the body and mind; it goes so far that when a person is interrupted during a sleep stage or completely skipped a stage, it can lead to sleep inertia (grogginess, impaired alertness and slower reflexes/responsiveness after awakening). Sleep experts have categorized sleep in four different sleep stages:
- Stage 1: beginning of sleep, the light sleep. Brain produces high amplitude theta waves.
- Stage 2: Transition from light to deep sleep, lasts about 20 minutes, brain produces sleep spindles (bursts of rapid, rhythmic brain waves), body heat starts to decrease as well as heart rate.
- Stage 3: Transition from light to deep sleep, deep and slow brain waves called delta waves are produced, this stage is also called delta sleep (named after the type of produced waves)
- Stage 4: Deep sleep, most dreams occur in this stage, also called REM sleep because of the eye movement during this stage. This stage is also called paradoxical sleep because even though the brain and other body systems are more active than before the muscles are actually becoming more relaxed during this stage and the body is essentially paralyzed.
Below is a table of these stages:
Sequence of stages/ sleep cycles
The process of sleep doesn’t actually follow the stages in succession. When sleep begins you will go into stage 1 and you will eventually go into stage 2 and 3. After stage 3 you will go into stage 2 again and after that into stage 4. After stage 4 is over you usually go back into stage 2. A sleep cycle is the progression of going from NREM to REM sleep. You go through these sleep cycles about four to five times during the night.
An average person enters stage 4 about 90 minutes after they fall asleep. In the first cycle NREM usually lasts longer than REM sleep; but with each succeeding sleep cycle the REM sleep lasts longer and the time in NREM of stage 3 decreases. REM sleep can actually last a whole hour in a later cycle. There are on average a total of four or five regular sleep cycles of NREM and REM sleep. The first sleep cycle has an average duration of 90 minutes the succeeding cycles are on average about 100-120 minutes long. Overall, during early sleep the stage 3 sleep lasts longer than stage 4; later in the night stage 4 lasts longer especially during the final two sleep cycles. Brain waves become slower, stronger and synchronized as you go to deeper levels of sleep.
Below is a table of the waves a sleep cycle follows and their states:
Resting wakefulness/(N)REM sleep
Light NREM sleep
Deep slow-wave NREM sleep
There is also an arousal threshold, or the ease with which the sleep can be disturbed (by noise or other external factors). This threshold will be higher at the deeper sleep cycles, so it’s more difficult to wake someone in stage 3 sleep, but relatively easy in stage 4 (REM) sleep. As the deeper sleep stages are reached the muscle tone decreases, especially during REM sleep.
Final words on sleep cycles
As we go through our daily lives it is very important to sleep well, because sleep is directly linked to our health and behavior during the day. Skipping sleep is not healthy and we should try to go through a full session uninterrupted. There are still many unanswered questions about sleep and the functions of each sleep cycle, but what we do know is that there is a good reason why we go through them.