Sleep is essential, but its core purpose is still unresolved. Several theories, likely overlapping, point to energy saving, bodily and brain restoration, and brain plasticity.
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Historian Roger Ekirch argues segmented sleep was the predominant pre-industrial Western pattern. Historian Niall Boyce has challenged that, and a study of three pre-industrial societies found no such pattern.
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No single explanation is settled. Memory consolidation is the leading theory, but emotion processing, threat rehearsal and 'no purpose at all' remain live alternatives.
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