How much sleep do we need? The answer might not be ‘a solid eight hours’

When I was young, sleep seemed simple; an unremarkable interlude between days. Throughout my adolescence and young adulthood, it increasingly became an obstacle to enjoying entire box sets, marathon video game sessions, and, for a brief and uncharacteristically adventurous period, all-night partying.

Subsequent periods of 4am work starts and overnight library sessions writing needlessly detailed essays left me with a sleep pattern so incoherent that it could have been mistaken for a US presidential candidate.

This is not a plea for sympathy. Like most people with young children I view earlier, self-inflicted patterns of disordered sleeping as a youthful indulgence, and the idea that I understood tiredness as charmingly naive. Now I exchange stories from the sleep regression trenches with fellow bleary-eyed parents, making hopeful noises about developmental leaps and wondering how I ever turned my nose up at a solid eight hours.

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How much sleep do we actually need? The idea of consolidated eight hours of sleep is a relatively modern phenomenon. Even today, different cultures and regions have different sleeping habits, but pre-industrial sleep patterns varied even more. In agricultural areas with intense midday sun, it was common to take an early afternoon break to escape the heat, to eat and to recharge. Some cultures embraced a brief power nap, while others observed a more extended break, incorporating leisurely meals and relaxation.

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Industrialisation, shift work and urbanisation all affected the traditional midday rest. Yet it remains culturally significant in some countries. In Spain, for instance, the working day often begins at 9am, features a lunch break of two to three hours, then ends as late as 7pm. As in many other countries, Spaniards traditionally went home for lunch. Now, with both adults usually working and commuting, this is less practical.

A quirk of history means the siesta has persisted in Spain. European working patterns generally changed after the second World War, as women entered the workforce and jobs moved into the industrial and service sectors. Spain, like Portugal, had shared a time zone with Ireland. In 1942, dictator Francisco Franco moved the clocks forward in solidarity with Germany and Italy.

The Pirahã do not have a consolidated block of sleep and instead takes naps lasting 15 minutes to two hours throughout the day

Spaniards now dine at the same solar time as the French and Italians, but the clock time is an hour later, as many northern Europeans who have arrived at a closed restaurant at 7pm will know. As a result, evening events and prime television often start around 10pm, embedding long late lunches and late finishes in the working day.

Some contemporary cultures that have retained pre-industrial lifestyles, such as the Pirahã people of the Amazon, offer clues about earlier sleeping habits. The Pirahã do not have a consolidated block of sleep and instead takes naps lasting 15 minutes to two hours throughout the day. There is less evidence about how early modern Europeans slept, but some accounts suggest segmented sleep was common. This biphasic pattern had two distinct cycles of sleep separated by an interval of wakefulness.

The first sleep began shortly after dusk and typically lasted until about midnight. People would then naturally awaken for an hour or so, which might be spent in prayer, meditation or reading. This period was a tranquil time for reflection or social interaction before the second phase of sleep, which lasted until dawn.

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With industrialisation, urbanisation and artificial lighting, traditional sleeping cycles were disrupted. In 1953 Nathaniel Kleitman and his graduate student Eugene Aserinsky identified rapid eye movement (REM) sleep, linked with vivid dreams and heightened brain activity. Kleitman and Aserinsky noticed that sleepers cycled through several phases. Disruption of this cycles seemed to cause disorders. This, alongside the identification of the internal biological cycle, the circadian rhythm, and sleep hygiene practices, meant that a single uninterrupted sleep was viewed as preferable, especially in societies with industrial or post-industrial working patterns.

However, in 1991 Thomas Wehr published the result of an experiment in which volunteers spent 14 hours each night in completely dark rooms for a month. By the end of the month, they had settled into a biphasic pattern of sleep, with two periods of four hours punctuated by a restful but wakeful interlude.

At the same time, historian Roger Ekrich was combing the archives for historical evidence of biphasic sleep and published his results in a 2001 book, At Day’s Close. Together, these suggest that without modern working patterns and artificial light, we would most likely not have a single eight-hour sleep. Whether that’s any comfort to my fellow exhausted parents is another story.

Stuart Mathieson is a postdoctoral fellow in the school of history and geography at Dublin City University