Sleep elasticity: accessory, useful and vital components of a single state

Giorgio F. Gilestro 

Sleep is universal, homeostatically defended and apparently costly, and for half a century this combination has been read as evidence of a vital, conserved function still awaiting identification. The search has not converged. Instead, it has accumulated a dozen candidate functions, each defensible but none commanding assent. I argue that the difficulty lies with the question rather than with the answers to it. Evolution optimises fitness, not function, and asking what fitness sleep confers, and to whom, removes the expectation of a single answer. Reviewing the comparative, experimental and epidemiological evidence, I propose the sleep elasticity hypothesis: that sleep is not one thing but up to three components of separate evolutionary origin, superimposed into a single state. An accessory component arose first as an ecological buffer, keeping an animal safely and economically inactive during the hours in which activity would not repay itself; it discharges no dedicated cellular transaction, yet carries most of the sevenfold interspecific variation in sleep duration, the elasticity that gives the hypothesis its name. A useful component comprises the lineage-specific processes subsequently exapted onto that pre-existing state, so that what a species does with its sleep becomes a fact about the species rather than about sleep. A vital component, if it exists, is a small and still unidentified residue, which I hold open as a hypothesis rather than deny. The three observations conventionally taken to establish indispensability, that deprivation kills, that sleep is costly, and that no sleepless animal exists, do not carry the weight placed on them, and the literature on the harms of sleep loss, in animals and humans alike, is confounded throughout by stress. Sleep elasticity instead accommodates what single-function accounts must explain away: the migrating bird that sheds sleep for weeks and never repays it, the cavefish that has lost most of its sleep but kept the homeostat that would defend it, the fur seal that abandons one sleep state at sea while defending the other, and the captive sloth that sleeps six hours longer than its free-living counterpart. What makes inactivity sustainable is the elevated arousal threshold, and depth measured that way, rather than the slow wave, offers the most promising route to separating the three components experimentally.