Continuing the serialization of my recent manuscript – “On Sleeping through the Night: Ecology, Economy and Ethics of a Vital Human Project.” – with the first main section.
1. The Peculiarity of Sleep as a Project
As far as we know, all – or very nearly all – animals spend some fraction of their lives in sleep. Humans typically sleep for eight hours out of every 24, generally through the period of darkness. Some animals require much less sleep, others much more: some species of bat may spend 20 hours out of every 24 in sleep (Rößler and Klein, 2024: 1149).
Sleep is not the absence of activity, but a period of altered activity. Research in recent decades has brought to light that sleep is a complex project or a configuration of projects with distinct stages, involving different degrees of immobility and disconnection. Terminology for the architecture of sleep varies somewhat, but one standard account distinguishes rapid eye movement (REM) sleep from nonrapid eye movement (NREM) sleep, with three stages of NREM sleep.
For a healthy human adult, the period of “sleep latency” – how long it takes to fall asleep – is typically less than 20 minutes. This is followed by NREM stage 1 (5-10% of “healthy” sleep), a “transitional stage” during which sleep may be “interrupted with the slightest noise.” NREM stage 2 (40-60%) is slightly deeper: “arousals require stronger stimulus than required in stage-1 sleep.” The third stage of NREM sleep (5-15%), often referred to as “slow wave sleep” or “delta sleep,” is the deepest (Johnson and Czeisler, 2022: 4).
Rapid eye movement (REM) sleep is associated with dreaming. For a healthy adult,
REM sleep occurs in four to six discrete episodes. During the first sleep cycle, REM sleep is short; however, it becomes longer across the night. The arousal threshold during REM is variable throughout the night. Brief episodes of wakefulness can occur during the sleep episode, typically near the REM transitions. (Johnson and Czeisler, 2022: 5)
A typical scientific account emphasizes that sleep is quite different from blank unconsciousness and even further removed from death: “its hallmark is a reversable disconnection from the environment, usually accompanied by immobility” (Tononi and Cirelli, 2014: 22). In this, modern scientific accounts are notably similar to Aristotle’s (1957: 454b25-28) description of sleep, at least on the surface: “the animal is defined by the possession of sensation, and we hold that sleep is in some way the immobilization or fettering of sensation, and that the release or relaxation of this is waking.”
Within certain limits, sleep is also a compulsory project for nearly all animals, and certainly so for human beings. We humans have some control over the time, place and manner in which we fall asleep; we can also attempt to stave off sleep in various ways (e.g., drinking strong coffee) but, if we fight it long enough, sleep will come and claim us anyway.
But why do animals sleep? As scientific inquiry into sleep emerged through the twentieth century, a broad consensus emerged that sleep serves a restorative function. In general terms, at least, “why we need to sleep seems clear: without sleep we become tired, irritable, and our brain functions less well. After a good night of sleep, brain and body feel refreshed and we are restored to normal function” (Tononi and Cirelli, 2014: 22).
The view that sleep serves a restorative function has not been without its detractors. In the 1970s, for example, Ray Meddis (1977: 131) observed that, despite the broad consensus, “there is no direct evidence to support the belief that sleep has any direct association with restitutive processes of any kind.” In the absence of such evidence, Meddis put forward the hypothesis that “the sleep instinct” is not a signal that the body’s systems need restoration, but rather a prompt to engage in behavioral strategy of seclusion and immobility. On this account, sleep is merely a vestige of an evolutionary past in which our forebears were especially vulnerable to predation or other mishaps in the dark of night.
Daniel Dennett (1995: 339) cites a similarly contrarian argument from Chet Raymo to the effect that sleep “lacks any biological function.” His response is intriguing:
But why does sleep need a ‘clear biological function’ at all? It is being awake that needs an explanation, and presumably its explanation is obvious. Animals – unlike plants – need to be awake at least part of the time, in order to search for food and procreate, as Raymo notes. But once you’ve headed down this path of leading an active existence, the cost-benefit analysis is far from obvious. Being awake is relatively costly, compared with lying dormant . . . So presumably Mother Nature economizes where she can. If we could get away with it, we’d ‘sleep’ our entire lives.
In short, the question may not be “Why do we sleep?” but “Why are we awake?”
The contrary views of Meddis and Raymo, and Dennett’s equally contrary rebuttal, are outliers. The consensus remains that sleep serves to restore physiological and neurological function, a project which cannot be carried out even through quiet restfulness.
Even so, one aspect of Meddis’ critique seems to hit home: even down to the present day, a precise and definitive account of the function of sleep remains elusive. One recent review of sleep research suggests that “why we sleep remains a mystery” (Simon et al., 2022) which is perhaps an exaggeration. More to the point, according to another recent review, sleep serves several distinct purposes, so it would not be “realistic” to identify a single function for sleep (Zielinski et al., 2016) (see also Tononi and Cirelli, 2014: 22).
Among the many possible functions of sleep are effects “related to the immune system, hormonal systems, thermoregulatory systems, and basic metabolic processes,” though the most significant consequences of sleep deprivation seem to be neurological and cognitive (Simon et al., 2022: 1).
In an account of the evolution of modern human cognition, Coolidge and Wynn (2018: 147-48) put forward the hypothesis that the transition from sleeping in trees to sleeping on the ground resulted in more consolidated and efficient sleep for Homo erectus and their descendants, which brought several benefits to human cognition:
(a) threat simulation, social rehearsal, and priming from the content of dreams; (b) creativity and innovation, also from the content of dreams, and (c) procedural memory consolidation and enhancement, particularly for visual-motor skills and visuospatial locations.
In sum, while there is not an overriding consensus on the purpose of sleep, it seems clear enough that the project of sleeping through the night serves a range of vital functions, most of which involve recovering from and preparing for the projects we pursue while we are awake.
Viewed from within the experience of someone settling down for the night, there is a further peculiarity of sleep as a project. Consider, for example, Merleau-Ponty’s (2012: 166) description of falling asleep:
I lie down on my bed, on my left side, with my knees drawn up; I close my eyes, breathe slowly, and distance myself from my projects. But this is where the power of my will or consciousness ends.
Notably, to fall asleep I must “distance myself from my projects,” setting aside the concerns and tasks of the day. If I succeed in suspending my other projects, sleep may come of its own accord.
Sleep ‘arrives’ at a particular moment, it settles upon this imitation of itself that I offered it, and I succeed in becoming what I pretended to be: that unseeing and nearly unthinking mass, confined to a point in space and no longer in the world except through the anonymous vigilance of the senses.
So, if falling asleep is itself taken as a project, one component of the larger project of sleeping through the night, it is a project that depends critically on not doing something, at least not consciously or voluntarily doing something. I may consciously go about the work of settling in for the night but then sleep will have to come – or not – of its own accord, even though it is my own brain that is covertly doing the work.
Merleau-Ponty (2012: 167) provides a further clue to the nature of sleep as a human project in his description: in sleep, I am “nearly unthinking,” not perfectly inert; the senses remain vigilant, but with higher thresholds of awareness. He continues:
This last link is surely what makes waking up possible: things will return through these half-open doors, or the sleeper will return through them to the world.
While much of the work of sleep occurs below the threshold of awareness, there remains a thread of connection with the world.
Sleep itself has a temporal thickness to it: I may wake with a sense of time having passed, perhaps even with a sense of what hour of the morning it is. For contrast, consider the experience of general anesthesia. Instructed to count backward from ten, I reach seven or perhaps six when, all at once, I am already emerging again into consciousness, however muddled that consciousness may be (see also Johnstone, 1973: 76).
So, to sum up, I set myself to the project of sleeping through the night and waking in the morning refreshed and ready to take up the projects of waking life. I cannot consciously choose to fall asleep, but must instead establish the conditions under which sleep may happen of its own accord. If I succeed, the complex architecture of sleep unfolds on its own, beyond my intention or intervention.
References
Aristotle (1957) On the Soul. Parva Naturalia. On Breath. Cambridge, MA: Harvard University Press.
Coolidge FL and Wynn T (2018) The Rise of Homo sapiens: The Evolution of Modern Thinking. Oxford: Oxford University Press.
Dennett DC (1995) Darwin’s Dangerous Idea: Evolution and the Meanings of Life. New York: Simon and Schuster.
Johnson DA and Czeisler CA (2022) Components of Normal Human Sleep. In: Nieto FJ and Petersen DJ (eds) Foundations of Sleep Health. Academic Press, pp.1-12.
Johnstone HW (1973) Toward a Philosophy of Sleep. Philosophy and Phenomenological Research 34(1): 73-81.
Meddis R (1977) The Sleep Instinct. London ; Boston: Routledge & K. Paul.
Merleau-Ponty M (2012) Phenomenology of Perception. New York: Routledge.
Rößler DC and Klein BA (2024) More Sleep for Behavioral Ecologists. Journal of Experimental Zoology: Part A Ecological & Integrative Physiology 341(10): 1147-1156.
Simon KC, Nadel L and Payne JD (2022) The Functions of Sleep: A Cognitive Neuroscience Perspective. Proc Natl Acad Sci U S A 119(44).
Tononi G and Cirelli C (2014) Sleep and the Price of Plasticity: From Synaptic and Cellular Homeostasis to Memory Consolidation and Integration. Neuron 81(1): 12-34.
Zielinski MR, McKenna JT and McCarley RW (2016) Functions and Mechanisms of Sleep. AIMS Neurosci 3(1): 67-104.
Leave a Reply