biphasic sleep

Before the Industrial Revolution, it was common for people to sleep in two stages (“dead sleep” and “morning sleep”, as they were often called), separated by an hour of wakefulness in the middle. It’s thought that the invention of artificial lights precipitated the loss of this intervening wakeful period, as people started staying up later and consolidating their sleep into a shorter block of time. At least one study has found that if people are placed into a situation where they’re living in darkness for 14 hours a day (with no natural light), they do indeed revert to a biphasic sleep pattern. The hour of wakefulness in the middle is usually described as “calmer” than our regular waking hours, or even “meditative”.

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