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Ultradian rhythm

Scientists believe there may be a brain rhythm that is mainly revealed during sleep as a 90-minute REM cycle. As this lasts less than 24 hours it is known as an ‘ultradian rhythm’ (Ultra from the Latin for ‘beyond’). Dreams occur on a roughly 90-minute cycle.

At the deep-sleep stage, the EEG shows highamplitude slow waves. The mind begins to drift, thoughts are unfocused, and the brain becomes ‘dormant’. A person in this stage of sleep tends to stop moving and their postural muscles relax. Heart rate and breathing slows, the body temperature ‘thermostat’ in the hypothalamus is lowered and body temperature reduced. Kidney function decreases and less urine is produced.