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Biphasic sleep is a sleep pattern that involves sleeping in two sessions during a 24-hour period.
You probably know someone who sleeps for half the night, gets up for an hour or two, then returns to sleep. Or you may sleep this way yourself. If so, you’re not alone. This type of sleep pattern is called a biphasic sleep pattern, and it’s both historically common and biologically natural.
Before electric lighting was common, history suggests that many people embraced a biphasic sleep pattern: they went to bed at nightfall, then spent several hours awake around midnight before returning to sleep for the rest of the night.
This type of sleep is surprisingly common: Research shows that up to 60 percent of adults may have two distinct sleep cycles per night, encouraging a biphasic-type sleep pattern.
Historically, the middle-of-the-night break between sleep periods was used for reading, study, eating, and sex. References to biphasic sleep are found in literature from the Renaissance era: In The Canterbury Tales, from the 14th century, one character announces she will return to bed after her first sleep. Virginia Tech historian Roger Ekirch says there are over 500 literary references to a two-phase sleep pattern before the Industrial Revolution. Other research suggests that when humans use only natural light, they may fall into a biphasic sleep pattern.
Today, some orders of monks and cloistered nuns follow a practice by which they wake in the middle of the night to pray. Called Matins, these regular prayers break up the night and essentially create a biphasic pattern for practitioners. Some practicing Muslims follow a similar pattern, waking in the middle of the night to pray, then returning to sleep until morning. Sleep studies show that people following these practices have healthy sleep cycles during these two sleep periods.
Biphasic sleep isn’t harmful to overall health. Many people with a biphasic sleep pattern get enough sleep and don’t experience daytime fatigue. However, midnight awakenings can cause anxiety if someone believes they have insomnia.
Some sleep experts believe that biphasic sleep patterns may lead to the diagnosis of “faux” insomnia–in other words, people with natural biphasic sleep patterns may mistakenly believe they have insomnia, and seek treatments they don’t need.