The sleep rhythm is the cyclical sequence of sleep stages, in which phases of light sleep are followed by regular phases of deep sleep, and several of the so-called non-REM stages are each concluded by a REM phase, in which a large part of the dreaming takes place. Via the sleep rhythm, the brain uses neurophysiological processes to ensure that the sleeper does not wake up prematurely and that sleep can thus continue for a longer period of time until a state of recovery is reached. The smallest deviations from the natural sleep rhythm can deprive sleep of its restfulness and make the affected person feel daytime sleepiness or lack of energy.Various disturbances of the sleep rhythm may even indicate specific sleep disorders such as narcolepsy or even other diseases such as heart failure.
What is the sleep rhythm?
Through the sleep rhythm, the brain uses neurophysiological processes to ensure that the sleeper does not wake up prematurely, allowing sleep to last longer until a state of rest is reached. The cyclical process in which a person’s sleep stages alternate is also called sleep rhythm or sleep cycle. In addition to the falling asleep phase, the different stages of sleep include the light sleep phase, the two deep sleep phases, and REM sleep, which is primarily for dream activity and information processing. All stages except REM sleep are also known as non-REM sleep. While humans sleep, deep sleep phases alternate with individual phases of light sleep at specific intervals. This variation in the depth of sleep is controlled by the brain, which ensures that the sleep state is maintained in this way. After the phase of falling asleep, the sleep process is subject to neurophysiological control. Towards the end of sleep, the individual sleep phases alternate at increasingly shorter intervals. The sleep rhythm thus changes according to the personal sleep volume until the sleeper wakes up. To be distinguished from the concept of sleep rhythm is the expression of the sleep-wake rhythm, which corresponds to the cyclical sequence of waking portions and sleeping portions per day.
Function and task
The cycles of sleep and the sleep rhythm in which a person enters them ensure sleeping through. During sleep the organs and cells of the body regenerate, but also the psyche regenerates and experiences and learned things are processed. For these reasons, sleep is vital for human beings and in order to ensure this vital process, the neurophysiological sleep rhythm is used. A healthy person goes through about four to seven sleep cycles per night, each lasting about 70 to 110 minutes. This sleep rhythm is also known as ultradian rhythm. It has the sleeper go through each of the non-REM stages N1, N2, and N3 followed by a repetition of stage N2. The repetition of the N2 stage is regularly followed by a REM phase. The more cycles the sleeper experiences, the more the deep sleep phase of these cycles decreases. In late cycles, for example, the sleeper usually no longer reaches any deep sleep phases at all, while the REM portion becomes increasingly higher in the morning hours. Thus, a healthy adult sleeps about five percent of the night in stage N1, up to 55 percent in stage N2 and up to 25 percent in stage N3. REM sleep also accounts for up to 25 percent of daily sleep, with the waking portion totaling about five percent. The values for the individual stages can be collected using polysomnography and help to create a sleep profile. Each sleep phase differs from the other in the speed of pulse rate, breathing and brain wave activity. Therefore, sleep laboratories can assess which sleep phase a patient is in by monitoring these and similar parameters.
Diseases and ailments
While the amount of sleep needed may vary from person to person, the sleep pattern in terms of sleep rhythm and the respective sleep stages remains the same regardless of personal sleep volume. Significant and chronic deviations from the natural sleep rhythm automatically make sleep less restful. Those affected thus often feel unrested or exhausted the next morning, lacking energy and unable to concentrate.Meanwhile, medicine also assumes that the sleep rhythm influences certain eating habits. The various symptoms resulting from disturbed sleep rhythms can occur even if the stages change only minimally in their sequence. A strong deviation from the percentages of the sleep stages can have disease value under certain circumstances. The same applies to interrupting waking reactions, which occur in particular in sleep apnea syndrome. In this disorder, tiny pauses in breathing occur during sleep stages, usually caused by extreme relaxation of the upper airway. For many other sleep disorders, however, REM sleep that begins too early is also characteristic, for example. REM stages shortly after falling asleep are also referred to as Sleep Onset REM periods, for example. These phenomena can be an indication of narcolepsy for sleep physicians, i.e. of sleeping sickness. In some cases, however, premature onset REM sleep also occurs in the course of sleep apnea syndrome. The sleep physician decides which sleep disorder is actually present by analyzing the entire sleep profile. Modern studies have for the first time been able to document a connection between heart function and sleep rhythm. For example, the sleep rhythm of people with chronic heart failure differs significantly from the sleep rhythm of healthy people. Cardiac dysfunction can have an effect, for example, as a reduced REM sleep fraction or a generally reduced light sleep fraction. There is also a connection between sleep rhythm and alcohol consumption. For example, the waking phases of alcoholics are said to significantly exceed the natural proportion of five percent.
