Wann findet der Tiefschlaf statt?

Hauptsächlich in den ersten 2–3 Schlafzyklen der Nacht, also in den ersten 4–5 Stunden. Wer spät ins Bett geht, verliert überproportional viel Tiefschlaf.

Wie viel REM-Schlaf brauche ich?

Etwa 20–25 % der Gesamtschlafzeit, bei 8 Stunden rund 90–120 Minuten REM-Schlaf.

Kann ich meinen Tiefschlaf erhöhen?

Ja: kein Alkohol, kühles Schlafzimmer (16–18°C), regelmäßige Schlafzeiten und Bewegung tagsüber steigern den Tiefschlafanteil nachweislich.

Was stört die Schlafphasen am meisten?

Alkohol unterdrückt Tiefschlaf und REM. Koffein verzögert den Einschlafprozess. Blaues Licht hemmt Melatonin. Schlafapnoe fragmentiert alle Phasen.

Sleep phases: REM, deep sleep & sleep cycle explained simply

Sleep feels like a single, unified state – but it is not. Every night, your body goes through several sleep cycles with clearly defined phases in which different biological processes take place. The Cleveland Clinic and the National Sleep Foundation have precisely described these phases.

Overview hide

The Sleep Cycle: 90–120 Minutes, 4–5 Times per Night

NREM Stage 1: Light Sleep – the Falling Asleep Phase

NREM Stage 2: Stable Sleep – where most of the action happens

NREM Stage 3: Deep Sleep – the most important regeneration phase

REM sleep: Dreams and emotional processing

Circadian rhythm: The internal 24-hour clock

What Disrupts Sleep Stages

Common Questions About Sleep Stages

When does deep sleep occur?

How much REM sleep do I need?

Can I increase my deep sleep?

Why do I sometimes remember dreams and sometimes not?

The Sleep Cycle: 90–120 Minutes, 4–5 Times per Night

A complete sleep cycle lasts between 90 and 120 minutes. In a typical 8-hour night, you go through 4–5 such cycles. Each cycle consists of NREM sleep (3 stages) and REM sleep.

Phase	Share of Sleep Time	Main Function
NREM Stage 1 (Light Sleep)	~5 %	Transition from Wakefulness → Sleep
NREM Stage 2 (Stable Sleep)	~45 %	Memory consolidation
NREM Stage 3 (Deep Sleep)	~25 %	Physical recovery, immune system
REM Sleep	~25 %	Dreams, brain processing, emotions

NREM Stage 1: Light Sleep – the Falling Asleep Phase

Stage 1 lasts only a few minutes and marks the transition from wakefulness to sleep. Heart rate, breathing rhythm, and brain activity begin to slow down. Muscle twitches (so-called hypnic jerks) are normal in this phase – the brain “checks” the body status one more time.

In this phase, one can be easily awakened and may feel as if they haven’t slept at all.

NREM Stage 2: Stable Sleep – where most of the action happens

With about 45 % of total sleep time, Stage 2 is the most common phase. The brain produces characteristic sleep spindles and K-complexes – both are protective mechanisms that prevent external stimuli from interrupting sleep.

According to the Cleveland Clinic, Stage 2 is central to memory consolidation: the brain sorts and catalogs information gathered during the day.

NREM Stage 3: Deep Sleep – the most important regeneration phase

Stage 3, also known as slow-wave sleep, is the deepest sleep phase. It occurs mainly in the first two sleep cycles – that is, in the first half of the night. Those who are awakened in this phase feel strongly disoriented.

What happens during deep sleep (according to Cleveland Clinic):

Tissue repair and cell regeneration
Release of growth hormones
Strengthening of the immune system
Energy reserves are replenished

Too little deep sleep – for example, due to alcohol, which has been shown to suppress deep sleep phases – leads to fatigue despite sufficient sleep duration.

REM sleep: Dreams and emotional processing

REM stands for “Rapid Eye Movement” – the eyes move quickly back and forth under the eyelids. The first REM phase lasts only about 10 minutes, later ones can last up to an hour.

What happens during REM sleep:

Most intense dream activity
The brain processes emotional experiences
Creative thinking and problem-solving are promoted
Muscles are temporarily paralyzed (sleep paralysis) – protection against acting out dreams

Sleep deprivation particularly shortens the REM portion – which explains irritability, worse memory, and limited emotional regulation.

Circadian rhythm: The internal 24-hour clock

The National Sleep Foundation emphasizes: sleep is not solely controlled by tiredness, but also by the circadian rhythm – an internal biological clock that runs on a ~24-hour cycle. Light is the strongest regulator: daylight promotes alertness, while darkness triggers melatonin release through the pineal gland.

Those who live against their circadian rhythm – for example, through shift work or irregular sleep times – disrupt the entire sequence of sleep stages.

What Disrupts Sleep Stages

Alcohol: Suppresses deep sleep and REM in the first half of the night
Caffeine: Blocks adenosine and delays the entry into stages 2 and 3
Blue light: Inhibits melatonin, shifts the sleep onset time backward
Sleep apnea: Fragments all stages through frequent awakenings
Stress: Elevated cortisol prevents descending into stage 3

Common Questions About Sleep Stages

When does deep sleep occur?

Mainly during the first 2–3 sleep cycles, that is, during the first 4–5 hours of the night. People who go to bed too late lose disproportionately more deep sleep.

How much REM sleep do I need?

About 20–25 % of total sleep time, so around 90–120 minutes for 8 hours. Lack of REM sleep is noticeable through emotional instability and poor memory.

Can I increase my deep sleep?

Yes: no alcohol, cool bedroom (16–18°C), regular sleep times, and physical activity during the day have been proven to increase the proportion of deep sleep.

Why do I sometimes remember dreams and sometimes not?

Dreams mainly occur during REM sleep. You only remember them if you wake up shortly after a REM phase. This happens more frequently in the second half of the night, when REM phases become longer.