SLEEP stages


Humans like other mammals express two types of sleep. These states have distinctive neurophysiological and psychophysiological characteristics.

  • REM (rapid eye movement)
  • NREM (non-rapid eye movement)

REM sleep derives its name from frequent burst of eye movement; it is also referred as paradoxical sleep because EEG during sleep is similar to that of waking. NREM sleep is characterized by decrease activation of EEG; it is also called as orthodox sleep.

Within REM and NREM, there are further classifications called stages. For clinical and research applications stages of sleep is characterized by visual scoring of three parameters: EEG, EOG and EMG. During wakefulness EEG show low voltage, fast activity. EMG has high tonic and phasic activity. During preparation for sleep alpha waves set in, and become prominent particularly in occipital regions.

NREM sleep is subdivided into four stages.

  • NREM-1
  • NREM-2
  • NREM-3
  • NREM-4

1.1 NREM-1

Sleep is usually entered through a transitional state, stage 1 sleep, characterized by loss of alpha activity and the appearance of a low voltage mixed frequency EEG pattern with prominent theta activity (3-7 cps) and occasional vertex sharp waves may also appear.  Eye movements become slow and rolling, and skeletal muscle tone relaxes.

Subjectively, stage 1 may not be perceived as sleep although there is a decreased awareness of

  • Sensory stimuli,
  • Particularly visual,
  • Mental activity becomes more dream-like.

Motor activity might persist and occasionally individuals experience sudden muscle contractions, and might have felling of falling down, dreamlike imagery and hypnic jerks.

1.2 NREM-2

After stage 1, sleep progress to stage 2 that is heralded by appearance of sleep spindles and k-complexes.

Sleep spindles: Sleep spindles are oscillations of 12 to 14 Hz with duration of 0.5 to 1.5 s. They are characteristic of stage 2 sleep.

K-complexes: High amplitude negative sharp waves.

      Alfred Lee Loomis has discovered K-complexes in 1939. He and his team had described k-complexes as positive-negative-positive waveform. K-complex consists of a brief negative high-voltage peak, usually greater than 100 µV, followed by a slower positive complex around 350 and 550 ms and at 900 ms a final negative peak.

      They described a number of features, such as that it usually follows auditory stimulation during sleep. The K-complex is of higher voltage in the frontal than in the central region. K-complex isn’t the result of activation of the primary sensory pathway. Alfred Lee Loomis and his team also presented the first evidence of K-complexes having a refractory period. The K-complex was thus unlikely to reflect a sensory event, for which the refractory period is much shorter. K-complexes can also be readily elicited by increases in inspiratory effort. Evoked K-complex technique is also used to assist in the localization of lesions in brain.

      They are created by the outward dendritic currents from the middle to the upper layers of the cerebral cortex. This is accompanied by a decrease in gamma wave activity. The activity of K-complexes is transferred to the thalamus where it synchronizes the thalamo-cortical network during sleep, producing sleep oscillations such as spindles and delta waves.

      Present research is in direction of deciphering the relationships between K-complexes, spindles, and delta EEG, the relationship between K-complexes and autonomic activation, the possible role of the K-complex as marker of stimulus processing during sleep, and the state of knowledge as to the underlying intracranial generators of the K-complex.


Fig1: Sleep spindle and K-complexes

Stage 2 and later stages of NREM and REM are perceived as sleep. It is followed by stage 3 & 4.

1.3 NREM-3 & 4

Also referred as slow wave sleep (SWS) delta sleep or deep sleep. Slow waves appear during these stages (ie) less than 2 cycles per second. Since arousal threshold increases gradually from stage 1 to stage 4 it is called as deep sleep. Eye movements ceases and EMG activity further decreases.

Stage-3: It requires a minimum of 20% and not more than 50% of epoch time occupied by slow waves.

Stage-4: It requires scores greater than 50% of epoch time occupied with slow waves.


Fig2: Sleep stages and EEG features


Fig3: Sleep stages and EEG features


Fig4: Sleep stages with respect to EEG,EOG and EMG


Fig5: Sleep stages and EEG features

Reference: Introduction to Sleep and Polysomnography James D. Geyer, MD Sachin Talathi, PhD Paul R. Carney, MD11

1.4 REM

REM sleep is not subdivided into stages, but is rather described in terms of

  • Tonic (persistent)
  • Phasic (episodic) components.
  • Tonic aspects of REM sleep include the activated EEG similar to that of stage 1, which may exhibit increased activity in the theta band (3-7 cps), and a generalized atonia of skeletal muscles except for the extra-ocular muscles and the diaphragm.
  • Phasic features of REM include irregular bursts of rapid eye movements and muscle twitches.


Stage Amplitude Frequency Waveform
Awake <50 15-50
Pre-sleep 50 8-12 Alpha
NREM-1 50-100 4-8 theta
NREM-2 50-150 4-15 Spindle
NREM-3 100-150 2-4 Spindle and slow waves
NREM-4 100-200 0.5-2 Slow waves and delta waves
REM <50 15-30


Mtech in Clinical Eng Jointly offered by Indian institute of technology Madras& Christian medical college Vellore& Sree chitra tirunal institute for medical sciences and technology Trivandrum.
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