Unihemispheric sleep, or half-brain sleep, allows specific species to maintain vital functions while resting. Cetaceans utilize this phenomenon to ensure constant movement for survival in aquatic environments, while birds employ it for panoramic threat detection, with sentinels at the edges of a flock keeping one eye open while the corresponding hemisphere remains alert. This process involves a rhythmic switching of hemispheres, ensuring both sides receive restorative non-REM sleep without compromising safety. Humans also exhibit a mild version of this behavior, often manifesting as the "first-night effect" in unfamiliar surroundings, where one hemisphere remains more vigilant than the other. Notably, this state of partial wakefulness is exclusive to deep non-REM sleep, as the brain has never been observed to split its activity during REM or dream sleep.
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