Vertebrates that never sleep: implications for sleep's basic function.

A major activity of the brain of most vertebrates during waking behavior is the processing of sensory information, preponderantly visual. This processing is not fully compatible with the brain's spontaneous oscillatory activity that maintains (refreshes) infrequently used circuits that store inherited and experiential information (memories). Great reduction in sensory input and processing during sleep permits the refreshment of memory circuits to occur unimpededly. Accordingly, sleep may have evolved as ever augmenting needs for processing visual information during waking behavior by brains of great complexity conflicted increasingly with needs to refresh memory circuits. The lack of a need for sleep by genetically blind fishes that live in caves, and sighted fishes that swim continuously, is consistent with this thesis, as their needs for processing of sensory information, predominantly visual, are either greatly reduced or nil. Reduced requirements for processing sensory information by continuously swimming fishes owe to the following aspects of their behavior and ecology: (1) visual input is greatly reduced or absent during lengthy periods of nocturnal activity; (2) schooling greatly reduces needs for sensory information, particularly visual; (3) being maintained through frequent use, circuitry for most inherited memories needs no refreshment; and (4) inasmuch as they lead a comparatively routine existence in essentially featureless, open waters, pelagic species acquire, and have need to refresh, relatively few experiential memories. Analogous circumstances could account for the ability of migrating birds to fly for days without rest or sleep.