Sleep stages
In healthy adults, sleep is characterised by recurring NREM and REM phases, each lasting approximately 90 to 110 minutes. Polysomnography defines these stages on the basis of measurable signals. Whilst the temporal distribution is well established, the same cannot be said for regenerative functions or rigid individual norms for specific phases.
Auch bekannt als: Schlafphasen, REM-Schlaf, Non-REM, Tiefschlaf, NREM
What the different stages of sleep refer to
Sleep is not a uniform state. In healthy adults, NREM and REM sleep typically alternate at intervals of around 90 to 110 minutes. A narrative review thus describes the general architecture of sleep, but does not automatically explain the function of each individual stage.
The interval is not an individual target value. The chronological sequence alone does not imply that each stage provides its own regenerative benefit, nor that a longer duration of a particular stage is automatically better.
How the stadiums are measured
In polysomnography, sleep stages are classified on the basis of several physiological signals. The official scoring standard classifies N2 on the basis of, amongst other things, sleep spindles and K-complexes; N3 on the basis of slow-wave activity; and REM on the basis of eye movements, EEG and low chin EMG activity.
These criteria answer the research question: What pattern is present in the sleep signal? They do not prove that N2, N3 or REM are necessary or sufficient for tissue repair, immune function, muscle growth or a specific cognitive function.
How the architecture is distributed throughout the night
In the adult sleep architecture described, slow-wave sleep (SWS) occurs predominantly during the first third of the night. REM sleep increases as the night progresses. During the final third of the night, light sleep and REM sleep predominantly alternate with one another.
This distribution explains why a night’s sleep does not consist of blocks of equal length. However, it does not imply either a fixed number of four to six cycles per night or a universal REM period between 5 and 7 am. Nor is there direct evidence to support the claim that waking up early inevitably shortens REM sleep in every person.
What a human study on cognitive functions reveals
A controlled sleep study involving young, healthy volunteers investigated a specific video-game-based problem-solving task. After an interval of around ten hours, 62 per cent of the sleep group and 24 per cent of the wakeful group solved the problem. This finding relates specifically to this task and this study design; it is not a general statement about creativity or about REM sleep alone.
In a second experiment, auditory-targeted memory reactivation was employed during SWS, REM or wakefulness. The solution rates were 56 per cent for SWS, 58 per cent for REM and 57 per cent for wakefulness. No additional effect of reactivation was found. This means that a general role for REM sleep in creativity has not been demonstrated.
What can be deduced regarding recovery and muscle building
There is no evidence to support the claim that N3 or SWS, in isolation, is a necessary or sufficient cause of tissue repair, immune restoration or muscle hypertrophy. The temporal clustering in the first third of the night makes the hypothesis of a functional division of labour plausible, but does not prove it.
There is a lack of stage-specific dose-response data regarding muscle protein synthesis, strength and hypertrophy. PSG stages are therefore measurement categories and not a measure of recovery. A single N3 or REM figure does not justify either a rigid individual target or a prediction of expected muscle growth.
Classification
The main factors under scrutiny here are the recurring alternation between NREM and REM, the rough distribution of SWS and REM throughout the night, the PSG criteria, and the task-specific sleep findings during problem-solving. A general classification of N2, N3 or REM as being exclusively associated with physical or mental recovery, respectively, goes beyond the evidence available. For training and muscle development, sleep architecture is therefore a contextual factor, but no substitute for directly measured performance or adaptation endpoints.
Sources
- Voss U. Functions of sleep architecture and the concept of protective fields. Rev Neurosci. 2004. PubMed
- American Academy of Sleep Medicine. Summary of Updates in v2.0 of the AASM Manual for the Scoring of Sleep and Associated Events. Open source
- Beijamini F et al. Sleep Facilitates Problem Solving With No Additional Gain Through Targeted Memory Reactivation. Front Behav Neurosci. 2021. PubMed
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