Massachusetts Institute of Technology via Science Daily:
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Sleep is usually considered an all-or-nothing state: The brain is either entirely awake or entirely asleep. However, MIT neuroscientists have discovered a brain circuit that can trigger small regions of the brain to fall asleep or become less alert, while the rest of the brain remains awake.
This circuit originates in a brain structure known as the thalamic reticular nucleus (TRN), which relays signals to the thalamus and then the brain’s cortex, inducing pockets of the slow, oscillating brain waves characteristic of deep sleep. Slow oscillations also occur during coma and general anesthesia, and are associated with decreased arousal. With enough TRN activity, these waves can take over the entire brain.