Sleep is a period of inactivity with raised thresholds to arousal by external stimuli. However, sleep is an active process. While some brain areas are inhibited others remain active. Measuring sleep Three physiological measures are normally taken to characterise sleep. Cycles earlier in the night are shorter and REM is more prominent in the second half of the night.
Researchers studying dreaming often use subjects who regularly have lucid dreams.
Neural mechanisms of sleep A number of brain areas and neurotransmitter systems are involved in sleep and dreaming. These include the brain stem reticular formation, raphe nuclei, basal forebrain area, and locus coeruleus. Neurotransmitters that affect sleep include serotonin, norepinephrine, dopamine and acetylcholine. The timing of the daily sleep-wake cycle or rhythm is controlled by a biological clock located in the suprachiasmatic nuclei of the hypothalamus. This circadian clock also synchronises or entrains our daily rhythms with the prevailing light-dark cycle.