In general, we divide our lives into waking and sleeping phases. While we can consciously control the phases of activity in the waking state, this is not readily possible in the sleep phase. The brain controls with a multitude of hormones and messenger substances those processes that switch the body active and inactive and keep it at that level for a certain time. Among many other scientists, especially somnologists Eugene Aserinsky and Nathaniel Kleitman described the phases of different activity levels in sleep and wakefulness. In this context, the latter developed the Basic Rest-Activity Cycle hypothesis, which refers to rhythmically alternating rest and activity phases.
What is the basic rest-activity cycle?
EEG (electroencephalogram) recording is the best way to record the brain‘s activity curve during a sleep phase, as most of the other functions it controls during wakefulness are reduced. In parallel, the autonomic nervous system influences brain activity by allowing or inhibiting its release of produced hormones. Thus, the brain can be induced to switch the body into activity mode or to let it rest. This basic cycle of “rest-activity” repeats itself over a period of one to two hours. Strikingly, this cycle regulates the body even when awake. The different phases of sleep are recorded and evaluated in hypnograms. This results firstly in the falling asleep phase with initial lying awake, secondly in the sleep phases N1, N2, N3 and (mostly) again N2, thirdly in the REM phase and fourthly, after several of these cycles, in the awakening after optimally several hours. Depending on the duration of sleep, an average of about six sleep cycles per night can be observed, which in turn last one to two hours.
Function and task
Sensory input is collected by the brain in short-term memory, filtered, and finally made available as long-term storage when necessary. The REM and non-REM phases are an important tool to “deposit” these memory contents in the brain in the right place. Rapid Eye Movement (REM) describes the heavy rolling of the eyes during the REM phase and is associated with intense dreaming. The libidinal behavior, such as hunger and sexual desire, is regulated as well as stress and concentration. The REM phase occurs just over halfway through the sleep cycle. This period is called REM latency and should not be permanently undercut. It is accompanied by the following phases:
Slow theta waves at the beginning of sleep phase N1 signal the brain’s readiness to be able or willing to fall asleep. Muscle tone decreases, for example, a sitter’s head drops to the chest or the arm slides off the table. The eyes begin to move slowly. So-called “K-complexes and sleep spindles” characterize the stable sleep phase N2. Here, the eye movements come to a standstill. Finally, during deep sleep N3, the EEG records extra long delta wave. Muscle tone and eye movements approach zero. The share of the N-phases in the sleep duration is about 75%, that of the R-phases about 25%. During the following cycles, the N3 phases decrease strongly in favor of the R phases. During the REM phase, in addition to the rapid eye movements that give it its name, there is also slightly increased blood pressure and increased breathing and pulse rates. Sodium and potassium are “consumed” in the brain in the waking state. After one to two hours (in children after about 50 minutes), their content drops to such an extent that concentration difficulties occur. This is followed by a phase of about 20 minutes in which hardly anything can be done. In parallel, the body rebuilds its reserves of potassium and sodium, and another cycle of high activity follows.
Diseases and ailments
The autonomic nervous system, also called the autonomic nervous system, signals fatigue states to the brain from all organs, including the blood (such as in disease) and muscles. The tissue hormone serotonin keeps the brain awake and is only active to a very limited extent in N3, while it disappears completely in REM. At the same time, in response to the signal from the suprachiasmatic nucleus, the pineal gland produces melatonin, which controls the length of sleep.The hypotalamus, at the behest of the formate reticularis, regulates the release of the hormone adrenaline from the adrenal medulla, which is responsible for maintaining tone and thus the waking state. In addition, the eye is connected to the hypothalamus and causes lowered production of orexin in darkness or closed eyelids, which is responsible for increased alertness in the waking state. From the above facts, there are a number of possible disorders in their interrelations and processes. Special nervous disorders concern uncontrollable leg movements and psychologically induced teeth grinding during sleep, which can lead to a disturbance in the deep sleep phase. Nightmares and illnesses also put an abrupt end to these phases, as do, for example, reflux esophagitis or respiratory arrest, which the body responds to with a reflexive wake-up signal. Excessive cortisol release from the adrenal cortex or a reduced hippocampus impair the necessary deep sleep phases. Dementia or depression can also be mentioned here as organic causes. Externally applied parameters such as alcohol, medication, caffeine and too little oxygen have an additional negative influence on healthy sleep.
