N3

N1

N2

REM

Wakefulness

Dreaming

Comprises approximately 75% of the sleep cycle

Is also termed synchronized sleep

Respiration becomes slower and more regular

Delta waves are recorded on an electroencephalogram (EEG)

Transition from wakefulness to sleep occurs

Dreams occur

Penile erections may occur

Blood flow to the brain is increased

Respirations become more rapid and less regular

rapid eye movement sleep

stage 1 non-rapid eye movement sleep

stage 2 non-rapid eye movement sleep

stage 3 non-rapid eye movement sleep

stage 4 non-rapid eye movement sleep

Pulse become slower and more regular

Blood flow to the brain is increased

Breathing is more irregular

Metabolic rate increases

Muscle paralysis normally occurs

Eye movements are slow and electroencephalogram (EEG) shows low brain wave activity.

Electroencephalogram (EEG) activity is increased, with the appearance of spikes called K complexes.

Eye movement ceases, wave frequency is reduced, and wave amplitude is increased.

Electroencephalogram records delta activity.

Deep sleep occurs.

Delta activity

Low brain wave activity

K complexes

Increased wave frequency

Decreased wave amplitude