Dental sleep medicine series 1: Normal sleep and sleep staging

When treating sleep in a dental setting, it is useful to have a basic understanding of sleep and sleep staging to be able to interpret the results of a sleep study correctly and to be able to communicate effectively with medical colleagues.

From Sleep Medicine 6th edition

What is sleep
Sleep is a reversible behavioral state of perceptual disengagement from and unresponsiveness to the environment. It is also true that sleep is a complex amalgam of physiologic and behavioral processes. Sleep is often associated with behavioural traits such as supine position, quiescence, closed eyes. Parasomnias can occur including sleep walking, sleep talking and tooth grinding.

What are the stages of sleep?
Two very separate stages of sleep have been identified: REM and Non REM sleep. NREM sleep is separated into 4 substages as defined by the EEG measurement axis (brain waves). NREM EEG is described as synchronous and have characteristics such as sleep spindles, K complexes, and high voltage slow waves. generally, the arousal threshold (difficulty of waking) increases with the increasing stages of NREM sleep 1 to 4. NREM sleep is associated with minimal or fragmentary mental activity. NREM sleep represents an inactive but actively regulating brain in a moveable body.

REM sleep is as different to NREM sleep as it is to wakefulness. It is characterised by EEG activation, muscle atonia (paralysis), cardiorespiratory irregularities and episodic bursts of rapid eye movement. REM sleep is associated with dreaming. Inhibition of spinal neurons by the brainstem causes suppression of motor tone. Therefore, REM is an active brain in a paralysed body.

Stages of sleep

Sleep onset
In normal circumstances in normal adult humans, the onset of sleep is through NREM. Abnormalities can indicate pathology e.g entry to sleep through REM can be a sign of narcolepsy. The measurement of sleep onset on PSG is not clearcut issue. A combination of EEG, EOG and EMG can be used to determine this as well as behavioural changes. Cessation of automatic behaviours, loss of visual and auditory perception, loss of olfactory perception (after stage 1 sleep), Hypnic myoclonia (generalised or localised muscle contraction often associated with vivid visual imagery. These are not pathologyic but are common in stress or unusual or irregular sleep schedules.)

Progression of sleep
A normal adult human enters sleep through NREM and REM sleep occurs about 80 minutes after. REM and NREM sleep alternate throughout the night with about 90 minute cycles.

NREM 1 sleep occurs for a few minutes after sleep onset. There is a low arousal threshold and the person can be easily awakened. It acts as transitional sleep throughout the night. A sign of pathology is increased percentage and occurrence of NREM1 sleep.

NREM2 sleep is brief and occurs for about 10-25 minutes in the first cycle. It is signalled by K complexes or sleep spindles in the EEG. The arousal threshold is higher than stage 1. often a stimulus that causes arousal in stage 1 only causes an evoked K complex in stage 2. As stage 2 progresses, high voltage, slow wave sleep appears in the EEG  transitioning into NREM3 sleep.

NREM3 sleep only lasts a few minutes in the first cycle and transitions into stage 4 sleep which usually lasts 20-40 minutes in the first cycle. Combined stage 304 sleep is known as  slow wave sleep, delta sleep, deep sleep or N3.

There is a brief ascension into lighter stage 3 then stage 2 sleep followed by the initial REM phase. The inital REM cycle is short <10 minutes and as is characteristic of REM sleep has a variable arousal threshold.

NREM and REM sleep continue to cycle throughout the night. REM sleep increases and stage 3-4 sleep decreases as the night progresses.

Across the night, stage 1 sleep will account for about 2% to 5%, stage 2 about 45% to 55%, SWS about 10% to 20%, and REM sleep about 20% to 25% of sleep in a healthy young adult.