Consciousness needs two things: arousal, which comes from the brainstem, and awareness, which comes from the cerebral hemispheres. Sleep is a normal, reversible drop in arousal with its own regular cycles.
Arousal system
The reticular activating system in the brainstem sends signals through the thalamus to keep the cortex awake. Damage to the brainstem, or to both hemispheres, can cause coma.
Sleep stages
Sleep cycles through non-REM stages, from light sleep to deep slow-wave sleep, and REM sleep, when most dreaming happens and the muscles go slack. One cycle lasts about 90 minutes and repeats four to six times a night.
Sleep and wake timing
The hypothalamus sets a 24-hour body clock that follows light and dark, and melatonin from the pineal gland rises at night. The pressure to sleep builds the longer we stay awake.
Altered consciousness
Confusion, drowsiness, stupor and coma are steps along one scale. Causes include low glucose, low oxygen, drugs, infection, seizures, stroke and head injury.
At the bedside
Score consciousness with the Glasgow Coma Scale (3 to 15) or AVPU and record the trend. Always check blood glucose in a patient with a reduced level of consciousness.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).