Which sleep cycle is the most important

The sleep cycle and its phases

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Our sleep runs in a repetitive sleep cycle that has to do important tasks for our physical and mental recovery. Although not all of the functions of sleep have been explored, one fact is rock-solid: sleep is essential for all human beings.

Sleep is divided into two forms: REM sleep and non-REM or NREM sleep. "REM" is the English abbreviation for "Rapid Eye Movement" and means "rapid eye movements" in German - one of the distinctive features of this sleep phase. REM sleep or “paradoxical sleep”, “discovered” in 1953, is also called the dream phase, as this is where dreams are most intense.

The phases of sleep

The sleep cycle is generally divided into the “lying awake” (W) stage and four REM phases: three NREM sleep phases and one REM sleep phase. The phases of NREM sleep are two lighter sleep phases, N1 and N2, and a deep sleep phase, N3. This classification was presented in a publication by the American Academy of Sleep Medicine in 2007 and has updated the older model by Rechtschaffen and Kales by combining both deep sleep phases into one deep sleep phase N3.

A closer look at the sleep cycle

Our sleep follows a certain profile: waking state, falling asleep phase with light sleep (interrupted by renewed waking phases), deep sleep and then, 60 to 90 minutes after falling asleep, an initial short REM sleep phase. This cycle is repeated several times a night, with the REM sleep phases extending over the course of the night for a duration of up to
to reach in an hour. During an 8-hour night's sleep, we go through 4 - 7 sleep cycles.

N1 - sleep phase

The sleep phase is a transitional or floating state between being awake and sleeping and is rarely longer than 15 minutes. The body comes to rest, breathing and pulse become more even, the muscles relax (in a sitting position, for example, the head falls forward), slow, rolling eye movements may occur. The EEG (measurement of brain wave activity) shows a slowdown in brain waves. When our brain is so relaxed that we can no longer perceive familiar noises (unknown things quickly wake up again) or gentle touches, we have fallen asleep.

N2 - stable sleep

After falling asleep, we move into the stable sleep stage. In this phase of the sleep cycle, muscle tension continues to decrease, breathing and heartbeat slow down, and there are no eye movements. This phase gets longer with each cycle and normally takes up about half of the total sleep time.
Since 2007, EEG measurements have ensured that the N2 phase occurs again after deep sleep and before REM sleep. This riddle has not yet been solved, but "N2 the second" is a fact and is sure to help our sleep.

N3 - deep sleep

Physical recovery, the regenerative phase, now begins. Blood pressure drops, heartbeat and breathing continue to slow, body temperature is lowest, muscle tension continues to decrease, and there are no eye movements. The body recovers and "repairs" itself, strengthens the immune system. The brain “learns” by repeating daily experiences. This is the “deepest” of the sleep phases, when it is most difficult to wake up. But right now, sleeping speaking and sleepwalking are occurring. With around half an hour per cycle (decreasing), deep sleep takes up 20 percent of the total sleep time with deviations.

R - REM sleep

In this phase the brain is very “awake” and there are activities that are similar to the waking state (“paradoxical sleep”). The eyes move quickly with the lids closed and there is an increase in heart and respiratory rate and blood pressure. Dreams are particularly intense, sexual arousal occurs and long-term procedural knowledge is strengthened (cycling, swimming, etc.). The skeletal muscles required for active movement are relaxed to such an extent that there is a real rigidity (REM atony). This “sleep paralysis” prevents intense dreams from being translated into movements and leading to injuries.

The length of the REM sleep phases increases with each cycle in contrast to the deep sleep phase. If the first REM phase lasts about 5 to 10 minutes, phases of more than 15 minutes can occur towards the end of sleep. Adults have a total REM sleep time of approximately one and a quarter hours. In stark contrast, newborns spend almost all of their sleep in REM sleep. Science concludes from this an important role in the development of the central nervous system.

Scientific information about sleep phases and how they are measured can be found here.