Usually associated with N.R.E.M. sleep cycles, these are bursts of neural activity in the brain during sleep. They are not well understood but are thought to be related to the neuroplasticity aspect of sleep where the brain integrates new knowledge from the day with older memory.
They seem to correlate with less stimulation from the outside world while the brain is asleep (as opposed to, for example, how the brain will take in stimuli from the outside world during sleep and incorporate them into dreams). Strange patterns of sleep spindles are often found in people with various sleeping disorders.
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