Did Humans Evolve to Sleep in One Long Stretch, or Was Segmented Sleep the Historical Norm? — Here’s What Researchers Suggest

In the modern world, someone having a great sleep is considered to be a long stretch of 7 to 8 hours at night, or monophasic sleep. So, for some people who wake up in between, before going back to bed again, also called biphasic sleep, it could be a sign that they did not get a good night’s sleep. However, there are some ongoing debates and studies that argue which sleeping patterns are more natural to us.

Could humans really have evolved to naturally sleep in one long stretch, or is segmented sleep also present in our history? Since both types of sleeping patterns actually have evidence that could support them, let’s look at some historical records, what the biology of being humans says, and some modern experiments to see what could be the norm for humans before the inventions of all these lights, schedules, and even industrialization.

What Historical Records Tell Us About Segmented Sleep

Humans did not always have all these lights around us, and we didn’t always have alarm clocks in our phones, and so on. There are a lot of inventions that the modern world has given us that could’ve affected our sleeping patterns. But, according to some historical records collected before the 1800s, humans may have slept closer to a segmented pattern than one long stretch.

Professor A. Roger Ekirch, who has done seminal research about sleep, has collected many references about a “first” and “second” sleep in various timelines across history, like ancient, medieval, and early modern. In old writings, diaries, and medical texts from different countries in Europe and beyond, this idea or practice was mentioned.

He found out that they would go to bed at sunset and sleep for 3 to 4 hours. This initial part is the one called the “first sleep”. Then, one would wake up in between, usually for an hour or two, and then return to bed for the “second sleep”, which was another several hours of sleep until dawn arrives.

The wakefulness period between the first and second sleep was considered normal. People would often use this time for some lighter activities, such as praying, reflecting on a dream they had, and even socializing in some instances. This quiet environment was also utilized by scholars, poets, and thinkers to do some of their tasks.

Read more: Harper’s Magazine

What Studies of Hunter-Gatherer and Non-Industrial Societies Suggest

Historical Sleep Patterns in Humans (Sleep and Dreaming)

To find out which sleeping pattern could be more natural, researchers also tested people in modern societies who still sleep like our ancestors used to. It refers to going to bed with fewer electric lights and with more exposure to the natural day and nighttime patterns, such as the sunrise and sunset.

A study published in Current Biology observed three different hunter-gatherer groups currently found in Tanzania, Namibia, and Bolivia. These people have limited exposure to artificial light, and they generally sleep closer to one stretch at night. However, they also got some brief and frequent awakenings at night, but not as long as the wakefulness period between the first and second sleep described in the mentioned historical texts.

This may be because the people living near the equator, like the hunter-gatherers in the study, had a consistent amount of darkness all year round and temperature cycles. While people who may have lived in Europe during their industrialization or before it, there was a longer period of darkness, and winters that could have naturally led individuals to segmented or biphasic sleeping patterns.

Another research was published by Professor Niall Boyce in 2023, which argues that the biphasic sleep described in the historical texts and the study of Professor A. Roger Ekirch may not be universal among the population of the world at the time. It may not even be the dominant pattern, because in many non-industrial and near-equator societies, people slept continuously in the night. It may be that the longer darkness in higher latitudes is the reason for the practice of the segmented sleeping pattern.

In other words, research studies found that humans may not have had one universal sleeping pattern that could be considered the natural way to sleep. But let’s explore these ideas even further.

Read more: Current Biology | PubMed Central

What Human Biology and Experiments Show

While historical texts may point to people having segmented or biphasic sleeping patterns, biological evidence and studies may further explain how it could be considered natural in a way.

Humans have an internal sleep-wake pattern called the circadian rhythm. It is influenced by light and darkness, most probably evolved from the cycle of sunrise and sunset. This knowledge may be important in analyzing other experiments that were done, such as the one by Thomas Wehr, a sleep scientist.

Thomas, in 1992, conducted an experiment that replicated the pre-industrial environment by confining participants in 14 hours of darkness per day for a month. What he found out was that after an initial period of adjustment, the participants began to sleep into a biphasic pattern, similarly to what the historical texts described.

They had a first sleep of about four hours, followed by a one to two wakefulness period, and then went back to sleep again. Some researchers interpret these findings as an evolutionary state. The one long stretch of sleep may be a forced rhythm influenced by our modern world rather than biology or what homo genus ancestors used to do in the past.

The amount of light one is exposed to could be affecting the intrinsic processes that control our sleep and circadian rhythm. However, it is best to acknowledge that we do not have any concrete evidence for what the early hominins’ sleeping patterns were thousands of years or even millions of years ago.

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What Might Be the Custom Today vs What Was Normal Before

Medieval Nights: The Lost Tradition of Two Sleeps

Today, most people consider a good night’s sleep as a long stretch of uninterrupted 7 to 8 hours of sleep. This could be influenced and shaped by the different aspects of the modern world, such as our culture, work, and school schedules, artificial lights, and devices.

However, in the past, before industrialization, people were more accustomed to darkness. According to historical records, sleeping earlier, waking up in the middle before going back to sleep again, was part of the norm.

Additionally, seasonal variations may have affected the sleeping patterns too, as longer nights in winter in higher latitudes may have increased the tendencies to biphasic or segmented patterns.

Moreover, after the industrialization period, the schedules of individuals became more fixed. People either worked in the dark or in the day for a continuous number of hours. Artificial lighting also spread, which could have pushed back bedtimes, and over time, the culture of consolidated sleep became idealized and expected.

Author's Final Thoughts

Researchers believe and have found evidence that biphasic or segmented sleeping patterns before modernization were part of the norm in society. However, whether it was the natural sleeping pattern of our ancestors or the Homo genus species is less certain.

Biology, historical records, and experiments suggest that humans can naturally divide sleep, especially in environments with less artificial lighting and longer periods of darkness. But today, most people sleep in one continuous stretch, likely because of how our modern culture, technology, and schedules shape our lives.

Read next: Why Did Humans Evolve to Sweat More Than Other Animals Around 2 Million Years Ago? — Here’s What Scientists Believe

References & Further Reading

Ekirch, A. R. (2001). Sleep we have lost: Pre-industrial slumber in the British Isles. American Historical Review. https://doi.org/10.1086/ahr/106.2.343

Ekirch, A. R. (2005). At Day’s Close: Night in Times Past. W. W. Norton & Company. https://wwnorton.com/books/9780393329018

Wehr, T. A. (1992). In short photoperiods, human sleep is biphasic. Journal of Sleep Research. https://doi.org/10.1111/j.1365-2869.1992.tb00019.x

Yetish, G., et al. (2015). Natural sleep and its seasonal variations in three pre-industrial societies. Current Biology. https://doi.org/10.1016/j.cub.2015.09.046