Glossary of Sleep-related terms

Stage 4 (NREM Stage 4, Delta Sleep, Slow Wave Sleep)

Stage 4 has similar attributes to stage 3, except there are a greater proportion of EEG delta waves. These high-voltage, slow EEG waves make up 50% or more of the record. This is the deepest stage of sleep; a more intense stimulus is needed to wake someone from stage 4 sleep than any other stage of NREM sleep. Young children have more stage 4 sleep and are much more difficult to arouse than adults. Both stages 3 and 4 decline with age and many older people report that their sleep is lighter than when they were young. Children typically spend 20-25% of sleep in stages 3 and 4 slow wave sleep; this decreases to less than 10% by age 60. Sleepwalking, sleep terrors, and sleep-related enuresis episodes generally occur in stages 3 or 4, or during arousals from this stage.1