Ethics Afield

The third main section of my recent manuscript, “On Sleeping through the Night: Ecology, Economy and Ethics of a Vital Human Project.”

3. The Ecology of Sleep: Affordances

Aldo Leopold (1949: 113) once encountered two young men canoeing on the Flambeau River, in northern Wisconsin. For the first time in their lives, he wrote, they were experiencing “the complete freedom to make mistakes,” away from all the “buffers” civilization had placed between them and the consequences of their actions. Notably, Leopold writes,

no friendly roof kept them dry when they misguessed whether or not to pitch the tent. No guide showed them which camping spots offered a nightlong breeze, and which a nightlong misery of mosquitoes; which firewood made clean coals, and which only smoke.

In the absence of the “buffers” of civilization, the choice of where to sleep was for those young men a matter of reading the landscape, discovering what it has to offer, and dealing with the consequences of their own decisions.

In other words, it was a matter of attending to the affordances of their environment, to employ a term coined by James J. Gibson. “The affordances of the environment are what it offers the animal,” writes Gibson (1986: 127), “what it provides or furnishes, either for good or for ill.”

Affordances are relative to the projects of an animal, supporting or thwarting them. Notably, ‘affordance’ also straddles the interior/exterior (or “subjective-objective”) distinction: it is both a “physical” feature of the world and a “psychical” feature of an animal’s perception of its surroundings. “An affordance points both ways, to the environment and to the observer” (Gibson, 1986: 129).

Here, then, is the ecological problem of sleeping securely. With what strategies does the animal fit itself and its projects into its environment? How does it deal with the negative affordances of its surroundings, and how does it take advantage of positive affordances?

Addressing the ecological problem of sleeping securely is partly a matter of adapting to our respective environments, fitting our bodies and our behaviors to what the landscape affords us.

For an early hominin sleeping on the ground, negative affordances might have included excessive light, excessive noise, exposure to adverse weather, unfavorable airflow, proximity to waste sinks, and the cross-cutting projects of pests, predators, and other people, including both out-group conflict and in-group betrayal. Positive affordances might have included darkness, concealment, natural shelter from weather (something “get-underneath-able” (Gibson, 1986: 38)), favorable air-flow, distance from waste sinks, and the presence of others whose projects align.

Some of the fit between animal and environment is already set as a product of evolution of each species, a matter of biological adaptation. “Sleep evolved in the natural world,” write Rattenborg, et al. (2017: 1), “shaped by environmental factors and ecological forces resulting in species-specific sleep architecture (i.e., the amount, composition, continuity and intensity of sleep).”

Hominins evolved as diurnal animals, with capabilities suited to activity during daylight hours. Relative to most waking projects, then, light is a positive affordance for hominins and darkness a negative affordance. Relative to the project of sleeping securely, though, the polarity is reversed: darkness becomes an advantage, affording concealment and the absence of stimuli that might cross the threshold of awareness and so disrupt sleep.

Other animals have adapted for very different niches. Some are nocturnal, some are crepuscular; some inhabit environments that would be fatal to others. The architecture of sleep varies accordingly. Cetaceans, for example, have been observed to engage in “unihemispheric sleep”– one hemisphere of the brain sleeps while the other remains awake – as a strategy for living as a mammal in an aquatic environment: “if your brain is only half asleep, you can surface to breathe” (Rößler and Klein, 2024: 1149). Meanwhile, elephant seals “enter full REM sleep in water, however they only do so at great depths of around 300m, likely in the absence of predators”  (Rattenborg et al., 2017: 1).

Sleep architecture in animals retains a degree of plasticity: it is a trait “that responds quickly to changes in local conditions, including the artificial laboratory environment” (Rattenborg et al., 2017: 1). This is to say that animals may exhibit behavioral adaptation, relatively flexible strategies for addressing the problem of sleeping securely in response to changes in the affordances of the landscape, or in response to movement to another landscape with different affordances.

Quiet stillness is a key adaptive strategy for sleeping securely. It is a strategy rooted in the architecture of sleep itself. Lima et al. (2005: 728) argue that, since neurological systems “must be taken off-line for maintenance and restoration,” falling into “whole-brain blackout” sleep may be the safest option for many animals. If parts of your brain are shut down for maintenance, it would probably be best not to go stumbling around in the dark! More than this, stillness during sleep affords concealment: an animal immobilized by “blackout” sleep, in the dark or otherwise concealed, and making little or no noise would be less likely than otherwise to draw the attention of predators.

In many animals, the immobility in sleep is supplemented by “cryptic behavior” in the period before going to sleep. Detailed study of the behavior of captive red-bellied tamarins confirmed that the monkeys became quieter and more vigilant in the half-hour before retiring to the nest box, reducing vocalizations even further after retiring (Caine, 1987: 242). Observations of two species of tamarin in the wild also supported this hypothesis, while also providing some insight into pressures on the selection of sleeping sites across the animals’ home range. The researchers add that “cryptic behavior prior to entering a sleeping site has been reported for a range of primate species” (Smith et al., 2007: 341).

Another strategy concerns site selection. Sorting out where to sleep involves a number of factors: predation pressure, proximity to food resources, defense of a home range, safety of the site, crowding, proximity to food resources, and the prospects for temperature regulation (Smith et al., 2007: 340; Lima et al., 2005: 730).  As with many animal behaviors, site selection is bound to involve trade-offs: “the safest sites will not necessarily allow for deep sleep. Under some circumstances, the safest available sites might be exposed spots where predators have trouble approaching unseen” (Lima et al., 2005: 730).

There is some evidence that some animals engage in a kind of “shell-game,” frequently moving to new sleeping sites to avoid predators (Lima et al., 2005: 730; see also Mitchell and Lima, 2002: 249-250). This may also involve a trade-off due to the “first-night effect,” such as many modern humans may experience when traveling: sleeping in an unfamiliar location for the first time (e.g., a hotel room) leads to different pattern of sleep, where one hemisphere of the brain is more alert than the other at a given time, which may make the entire sleep period less restful overall (Samson et al., 2017: 6). In the absence of familiarity, trust gives way to heightened vigilance.

Aside from behaviors on either end of sleep, vigilance of various degrees is built into the architecture of sleep for many animals. Each animal in a population has a distinct ‘chronotype,’ a variation of the architecture of sleep in terms of the timing and duration of each stage. For a given group of humans, chronotype will vary from person to person such that someone is likely awake or in a more vigilant stage of sleep at any given time of the night – or so goes the sentinel hypothesis.

A recent study of the Hadza, a hunter-gatherer group living in Tanzania, provided evidence in support of the sentinel hypothesis. Using a sleep survey administered in Swahili, and monitoring sleep patterns through the use of smart watches, Samson, et al.,

found that inter-individual variation in chronotype and periodic awakenings are sufficient to generate consistent sentinel-like behaviour throughout the night for Hadza hunter-gatherers, without the need for any active behavioural mechanisms, like the posting of actual sentinels, to maintain asynchrony (Samson et al., 2017).

Even in the absence of a formal, organized watch, the group may be vigilant even when the individual is not.

Sleeping together in groups also has its disadvantages. A larger group would have a higher risk of drawing the attention of predators, for example. Density also exacerbates the problem of sleeping in proximity to waste sinks, with the attendant problems of hygiene. Some primates reduce risk of exposure to pests and parasites by avoiding accumulation of feces under their sleeping sites; they may accomplish this by shifting sites frequently or even by building nests over running water (Anderson, 1998: 67-68).

Another strategic consideration in site selection and preparation involves attention to airflow, both in relation to insects and other pests and for regulation of body temperature. An example of this comes, once again, from Aldo Leopold, who seems unusually attentive to the flow of air.

Leopold (1949: 90-91) ventures that chickadees are not subject to much predation pressure but are subject to severe constraints in maintaining the temperature of their small bodies. He muses: “it seems likely that weather is the only killer so devoid of both humor and dimension as to kill a chickadee.” He goes on to offer a fanciful account of “chickadee Sunday school” and the two commandments taught there: “thou shalt not venture into windy places in winter” and “thou shalt not get wet before a blizzard.”

He saw that second commandment in action as he watched a band of chickadees settling in for the night in drizzly winter weather. The rain was coming from the south as the chickadees went to roost, but Leopold foresaw that the wind would change and the temperature drop overnight. The birds “went to bed in a dead oak the bark of which had peeled and warped into curls, cups, and hollows of various sizes, shapes, and exposures.” Due to the coming change in the wind, the affordances of each warp in the bark would change overnight: “The bird selecting a roost dry against a south drizzle, but vulnerable to a north one would surely be frozen by morning. The bird selecting a roost dry from all sides would awaken safe. This, I think, is the kind of wisdom that spells survival in chickdom.”

References

Anderson JR (1998) Sleep, Sleeping Sites, and Sleep-Related Activities: Awakening to Their Significance. American Journal of Primatology 46(1): 63-75.

Caine NC (1987) Vigilance, Vocalizations, and Cryptic Behavior at Retirement in Captive Groups of Red-Bellied Tamarins (Saguinus labiatus). American Journal of Primatology 12(3): 241-250.

Gibson JJ (1986) The Ecological Approach to Visual Perception. Hillsdale, New Jersey: Lawrence Erlbaum Associates.

Leopold A (1949) A Sand County Almanac and Sketches Here and There. New York: Oxford University Press.

Lima SL, Rattenborg NC, Lesku JA, et al. (2005) Sleeping Under the Risk of Predation. Animal Behaviour 70: 723-736.

Mitchell WA and Lima SL (2002) Predator-Prey Shell Games: Large-Scale Movement and Its Implications for Decision-Making by Prey. Oikos 99(2): 249-259.

Rattenborg NC, de la Iglesia HO, Kempenaers B, et al. (2017) Sleep Research Goes Wild: New Methods and Approaches to Investigate the Ecology, Evolution and Functions of Sleep. Philosophical Transactions of the Royal Society B: Biological Sciences 372(1734): 1-14.

Rößler DC and Klein BA (2024) More Sleep for Behavioral Ecologists. Journal of Experimental Zoology: Part A Ecological & Integrative Physiology 341(10): 1147-1156.

Samson DR, Crittenden AN, Mabulla IA, et al. (2017) Chronotype Variation Drives Night-Time Sentinel-Like Behaviour in Hunter-Gatherers. Proceedings of the Royal Society B-Biological Sciences 284(1858).

Smith AC, Knogge C, Huck M, et al. (2007) Long-Term Patterns of Sleeping Site Use in Wild Saddleback (Saguinus fuscicollis) and Mustached Tamarins (S. mystax): Effects of Foraging, Thermoregulation, Predation, and Resource Defense Constraints. American Journal of Physical Anthropology 134(3): 340-353.