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Sleep is no longer thought of as merely the time we spend unconscious. It is a dynamic state characterized by shifting levels of electrical activity and the ebb and flow of chemicals into various regions of the brain. Key to this give-and-take are two tiny structures in the hypothalamus deep in the brain. The neural dance they engage in determines when we fall asleep and when we wake again to face the day.

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Sleep depends on a pinhead-size cluster of cells called the ventrolateral preoptic nucleus (VLPO). Triggered by the daily up of the chemical adenosine, the VLPO sends a signal to arousal centres to stop producing histamine and other chemicals that keep us alert.

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Awakening is initiated by the body’s master biological clock, located in another tiny cell cluster called the suprachiasmatic nucleus (SCN). Responding to light, the SCN generates a “wake up” cue that signals the VLPO to stop firing, reactivating the arousal system

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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.

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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 working hours

Retina:

Contains special cells that send an arousal signal to the brain when they sense light