For most of human history, sleep was understood as a passage — a nightly migration from the waking world into a realm that, while mysterious, was no less real and no less consequential than the daylit landscape the sleeper had left behind. Dreams were omens, visitations, or voyages; the sleeping body was a vessel temporarily vacated by a soul that had business elsewhere. This conception of sleep as a meaningful transit between ontologically distinct domains persisted, in various forms, from the earliest recorded civilisations through the European Renaissance, and its residue can still be detected in the language we use: we speak of 'falling' asleep, as though consciousness were a ledge from which one topples into an abyss, and of 'waking up,' as though morning were a vertical ascent back to the surface of the real.
The modern reconceptualisation of sleep as a biological function rather than a metaphysical event began in earnest with the development of electroencephalography in the 1920s, which made it possible, for the first time, to observe the sleeping brain in the act of sleeping. What the early electroencephalographers discovered was not a brain that had shut down — the popular metaphor of sleep as a kind of neurological hibernation — but a brain engaged in patterns of activity as complex and as highly organised as anything observed during wakefulness. The subsequent identification of distinct sleep stages, culminating in the discovery of rapid eye movement sleep in 1953, confirmed that sleep was not a unitary state of diminished consciousness but a structured sequence of neurologically distinct phases, each with its own characteristic architecture and, presumably, its own functional purpose.
The question of what that purpose might be has proven remarkably resistant to definitive resolution. The sheer universality of sleep — every animal species that has been studied sleeps, despite the enormous evolutionary cost of periodic vulnerability to predation — argues powerfully that it serves some function so vital that natural selection has been unable to dispense with it. Yet the nature of that function remains a matter of vigorous scientific debate. The hypotheses that have been advanced are numerous and not mutually exclusive: sleep consolidates memory, restores metabolic equilibrium, facilitates synaptic pruning, supports immune function, clears neurotoxic waste products from the brain. Each of these proposals is supported by experimental evidence of varying quality, and it is entirely possible that sleep serves all of these functions simultaneously, which would explain why evolution has proven so unwilling to economise on it.
What is most striking about contemporary sleep science, however, is not the sophistication of its methods or the ingenuity of its hypotheses but the impoverishment of the conceptual framework within which it operates. By reducing sleep to a set of measurable physiological processes, modern science has gained enormous explanatory power at the cost of rendering invisible precisely those dimensions of the sleeping experience that earlier cultures regarded as most significant. The dream, which for millennia stood at the centre of human reflection on sleep, has been relegated to the status of an epiphenomenon — a neurological by-product of memory consolidation, interesting perhaps as a diagnostic tool but devoid of the intrinsic significance that every pre-modern culture attributed to it.
This reductionism has practical as well as philosophical consequences. The contemporary epidemic of sleep disorders — insomnia, hypersomnia, the various parasomnias that afflict an increasingly large proportion of the population — is typically addressed through pharmacological and behavioural interventions designed to optimise sleep's measurable parameters: its duration, its latency, its architectural composition. What these interventions cannot address is the experiential quality of sleep — the sense, reported by many sufferers, that their sleep, however statistically normal, has become shallow, unrestorative, somehow emptied of the regenerative depth it once possessed. It is at least worth considering whether this impoverishment reflects not a failure of the biological mechanism but a consequence of the cultural disenchantment of sleep itself — the possibility that in stripping sleep of its meaning, we have inadvertently diminished our capacity to do it well.