National Geographic : 2010 May
THE ANATOMY OF SLEEP Hypothalamus: Critical to sleep; contains clusters of neurons that govern circadian rhythms and regulate chemicals promoting sleep and arousal Thalamus: Blocks input from the senses, allowing the brain to focus on processing information from the day Pineal gland: Produces melatonin when the body's clock senses darkness, helping the brain prepare for sleep Hippocampus: Vital to memory formation; during REM sleep, replays memories to be stored Pons: Involved in both arousal and the activation of dreams; during REM sleep, blocks signals to the spinal cord, preventing us from acting out our dreams Cerebral cortex: Activated during REM sleep by signals from the pons; dreams may be the cortex's attempts to create a "story" out of information collected during waking hours Retina: Contains special cells that send an arousal signal to the brain when they sense light STAGES OF SLEEP At night we cycle several times through ever deeper phases of sleep. In stage 1 (light sleep) we may drift in and out of wakefulness. Brain waves slow in stage 2, with occasional bursts of rapid waves. Stage 3 (split into 3 and 4 by some) is deep sleep, with extremely slow brain waves. More active periods of REM (rapid eye movement) sleep punctuate the stages: Heart rate and breathing grow more rapid; most dreams occur. Brain stem Thalamus Retina Pons Cerebral cortex Hypothalamus Hippocampus Pineal gland REM Low High BRAIN ACTIVITY DURING SLEEP Non-REM Adult Sleep Awake Stage 1 REM Stage 2 Stage 3 Midnight 1 a.m. 2 a.m. 4 a.m. 6 a.m. 6:40 a.m. 3 a.m. 5 a.m.