The amount of sleep and daily timing has changed throughout human history, whereas historical records shows that bed time sleep at night overlapped the arrival of dusk, modern society has created a sleep schedule that is highly influenced by artificially lit environments.
Light, whether natural or artificial, is among the most important environmental factors regulating sleep. In humans, light induces alertness and modifies a master circadian clock (biological clock) that regulates the timing of sleep 1 . Thus, the ability to control our own exposure to light through artificial means, has likely altered when we sleep and for how long.
The Romans, Greeks and Incas were guided naturally by the stars, sunrise, birds singing and agricultural needs. Up to the 14th century, sundials and hourglasses were used, only to be replaced by mechanical clocks. By the 19th century many individuals were wearing clocks, allowing them to schedule appointments, dinners or sleep hours.
Before the industrial revolution and before artificial lighting was widely used, segmented sleep was the dominant form of human sleep in Western civilization. It is characterized by sleep in 2 shifts 2 , called the “first sleep” and “second sleep.” In those times, sleep was more closely associated with sunset and sunrise than it is now. Within an hour or so after sunset, people retired to bed, slept for about 4 hours and then woke up. They remained awake for a few hours and then returned to sleep at about 2 am for another 4 hours or so. Written records from before the first century onward indicate that the period between first and second sleep afforded persons a chance for quiet contemplation, but persons also rose from bed during this interval and did household chores or visited family and friends. Wealthier families who could afford artificial lighting were able to go to bed later, thus usually having a continuous sleep.
The first and second sleep started to disappear during the late 17th Century. This started among the urban upper classes in northern Europe and over the course of the next 200 years filtered down to the rest of Western society. When the lamp was introduced in the late 19th century, everything changed. Artificial lightning extends the day and changed our nights, and then our sleep.
In an experiment conducted by Thomas Wehr, eight healthy men were confined to a room for 14 hours of darkness every day for an entire month. At first the participants slept for about 11 hours, presumably making up for their sleep debt. After this the participants began to sleep much as people in pre-industrial times had. They would sleep for about 4 hours, wake up for 2-3 hours, then go back to bed for another 4 hours. They also took about 2 hours to fall asleep 3 .
Following this experiment, Wehr concluded that this type of segmented sleep (called biphasic sleep) is the most natural sleep pattern, and is actually beneficial rather than being a form of insomnia. He then added that modern humans are chronically sleep-deprived, which would explain why we usually take only 15 minutes to fall asleep and why we avoid waking up during the night.