Although sleep is a need, it is a negotiable one. Societies have historically varied in their practices, and minorities have got by on less than is needed, sometimes for a lengthy period. Everyone weighs sleep against other priorities. This plasticity is what gives scope for the individual to be exploited, by herself and others, as sleep’s nonessential component is eroded. Sleep inequalities persist because their consequences can be deferred – but they cannot be deferred indefinitely.

Short sleep is often coupled with irregular sleep. For a substantial minority, hours of rest change in quick succession. They involve departures from night-time sleeping, and inconstancy and uncertainty in location. The service sector offers many examples.

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Increased risk of accident is one of the more dramatic effects of short and irregular sleep. Two in five UK doctors have reported falling asleep at the wheel, and some are known to have died. Among the other disadvantages associated with poor sleep are obesity, infection, psychological illness, attention deficit, and weak memory. Bad sleep also compounds unrelated problems. It affects a person’s mood, and thereby their evaluation and experience of what they encounter. It can heighten disadvantages to do with class, race, gender, as well as people’s appraisal of the prospects for changing their circumstances. Less privileged groups tend to have less regular sleep, less control over its timing, less motivation to protect it, and less capacity to set the norms by which society lives.

One may speak of a ‘sleep cleavage’, dividing the deprived and the rested. Those on the wrong side of these inequalities are not just those who sleep shorter than average hours, but those with particular needs.

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Insofar as sleep is discussed in public policy today, it tends to be framed as a matter of self-care. Individuals are encouraged to make good choices. The pattern is replicated in public debate, as commentators recommend practices of ‘sleep hygiene’, such as aiming to go to sleep around 10pm. As an approach to circadian justice, this seems wholly insufficient. Such responses tend to privatise sleep and its discontents. Holding individuals responsible for collective problems is generally a bad idea, but especially in an area like sleep, where feelings of personal responsibility generate added anxiety.

Read more in Aeon.