In the past, the night was divided in two, but society changed our sense of time

Coherent night sleep is a modern invention, not a evolutionary constant.

This may explain why many of us still wake up at three in the morning and wonder if something is wrong.

Coherent night sleep is a modern invention, not a evolutionary constant.

This may explain why many of us still wake up at three in the morning and wonder if something is wrong.

It can be a comfort to know that it is a deeply human one experience. For the vast majority of human history, eight hours was uninterrupted night sleep not the norm. Instead, you typically slept in two shifts every night, often called 'first sleep' and 'second sleep'. Each period lasted several hours, separated of a waking moment of an hour or more in the middle of the night.

Historical sources from Europe, Africa, Asia and elsewhere describes how families went to bed early after dark, woke up around midnight for a while and then slept on until dawn.

The middle interval was not wasted time

This division of the night probably changed ours experience of time. The quiet interval gave the night a clear center point, which could make the long winter evenings less endless and easier to handle. The midnight interval was not wasted time, but 'observed' time, which characterized how the long nights were experienced.

Some got up to take care of chores like stirring up the fire or look at the animals. Others stayed in bed to pray or reflect on the dreams they had just had.

Letters and diaries from pre-industrial times describe how people spent the quiet hours reading, writing or even enjoying themselves quietly and quietly with family or neighbours. Many couples also took advantage of this nocturnal wakefulness for a little lovemaking.

Literature dating back to the ancient Greek poet Homer and the Roman poet Vergil contains references to “an hour that ends the first sleep', which shows how widespread the two-part night was.

How we lost the 'second sleep'

The other sleep disappeared over the past two centuries as a result of profound social changes.

Artificial light is one of the main causes. In the 1700s and The 1800s first saw the introduction of oil lamps, then gas lights and finally electric lights to turn the night into usable waking time. Instead of going to bed soon after sunset, people began to stay up late into the evening in the glow of lamplight.

Biologically speaking, the strong light in the evening also changed our internal clock (our circadian rhythm) and made the body less likely to wake up after a few hours of sleep.

The timing of light matters a lot. Ordinary light before bedtime suppresses and delays melatonin, pushing sleep later.

The industrial revolution not only changed people's work, but also their sleep. The factories' schedules called for one unified sleep block.

Two periods of sleep come naturally to us

In the early 1900s, the idea of eight uninterrupted hours replaced the centuries-old pattern of two sleep periods. But in longer-term sleep studies, where you simulate long winter nights in the dark and remove clocks and artificial light in the evening, participants often end up falling back to a two-part sleep with a quiet wake interval.

A 2017 study of a farming community in Madagascar without electricity showed that people there still mostly slept in two segments and stood up around midnight.

Light controls our internal clock and affects how fast we move experience time passing. When these signals weaken – like in winter or during artificial light – it begins to flow.

In winter, later and weaker morning light makes it more difficult to keep the circadian rhythm. Morning light is particularly important for regulating the circadian rhythm, because it contains a greater amount of blue light, which is the most effective wavelength to stimulate the body's production of cortisol and suppress melatonin.

In time-isolated laboratories and in cave experiments, people have lived for weeks without natural light or clocks – some even in constant darkness. Many subjects were wrong about how many days had passed, showing that how easily our sense of time slips when the light signals disappear.

People from high latitudes handle the Arctic winter

Similar shifts occur during the polar night, where the absence of sunrise and sunset can make it feel like time stops does not exist. People born and raised in high latitudes, and residents with regular routines often handle polar light conditions better than visitors, but the effect varies by population and context. It helps when an entire society shares a stable circadian rhythm.

A 1993 study of groups of Icelanders and their descendants who had immigrated to Canada showed an unusually low incidence of seasonal depression (SAD).

The researchers suggested that genetic factors may help this population to cope with the long arctic winter.