A split of the nighttime sleeping period into two halves seems fairly common. You probably know people even today who sleep like this: sleep regularly for half the night, get up and do things for an hour or two, and return to sleep for the second half of the night.
The philosophy behind a bi-phasic sleep schedule sounds more grounded than that of a polyphasic sleep schedule, partly because most advocates and practitioners of bi-phasic sleep don’t try to cheat biology and cut the total amount of sleep.
Indeed, many have suggested biphasic sleep is a more natural pattern than sleeping through the night. And that in pre-industrial times people slept this way. There is anecdotal evidence of this; it appears that even in the days before electricity, some people slept the night through while others woke for a couple hours in the middle of the night for revelry. Virginia Tech historian Roger Ekirch says there are over 500 literary references to a two-phase sleep pattern before the Industrial Revolution. A researcher at the National Institute of Health attempted to find out the natural sleep pattern of humans and found that when left along with natural lighting, people did indeed fall into a biphasic pattern.
Physiologists are always trying to find mathematical models of the sleep process, and scientists at Harvard Medical School came up with an intriguing one to explain internal desynchrony. It involves known facts about the brain's hypothalamus and brain stem. Read about it here. The interesting here is that the model is said to predict the transition to biphasic sleep, both from uniphasic sleep and from irregular sleep wake rhythm disorder.
If people think they are supposed to sleep through the night and find themselves awake in the middle of the night for an hour, they might think they have sleep maintenance insomnia. But if they have no daytime sleepiness on a regular basis, they do not meet the formal criteria for insomnia. Maybe they just have a natural biphasic sleep pattern, and that rather than take drugs to sleep continuously, they should embrace this natural pattern. Writing in the New York Times, psychiatrist Richard Friedman bemoaned his patients who think they have insomnia when they just have a bi-phasic pattern.
Physiologists have noted that the sleep architecture of a significant portion of the population shows a definite split between two long periods of NREM sleep. Measurement of core body temperature show a biphasic pattern in some studies, especially in people who follow a biphasic sleep pattern habitually. Whether the body has "learned" to follow this temperature pattern or the sleep habit reinforces an existing tendency is not known.
Although some people can sleep through the shallow portion in the middle of the sleep period, others wake up and fit more naturally into a biphasic pattern. Children and adolescents rarely use a biphasic pattern, but anecdotal evidence show it is reasonably common in adults. We could not find any formal surveys on this topic, but we estimate a quarter of adults experience biphasic sleep. Is the waking period in the middle of the night the inverse of a nap – an anti-nap? You could see if that way. Why don’t we have a word for nighttime awakenings as we have for daytime "naps"?
Some orders of monks and cloistered nuns follow a practice by which they wake in the very early morning (or middle of the night) to pray. Called Matins, these regular prayers break up the night and essentially create a biphasic pattern for practitioners. Polysomnographic analysis of people who have followed this practice for years shows well shaped sleep architectures during their two periods. Some observant Muslims regularly say the Fajr prayer at dawn and then go back to sleep until they get up to go to work. A study in Saudi Arabia looked at the efficiency of sleepers who followed this practice versus those who slept straight through the night and found those who used the biphasic system were able to achieve the same efficiency,
At high latitudes, the nights are long during the winter – longer than the normal length of time most humans need during a 24-hour period. And indeed most people living in these areas did not sleep during the entire dark period before artificial lighting. A bi-phasic pattern was apparently common in Northern Europe centuries ago. The middle waking period was often used for meditation and prayer. The Canterbury Tales (14th Century) includes one character who during the night announces she will return to bed after her first sleep. In a fairly well-known experiment, Thomas A. Wehr at the National Institute of Mental Health put modern Americans in an environment with 14-hour dark periods (simulating winter nights) and found they adopted a bi-phasic pattern over time with no prompting from him.
There is also a tradition of sexual activity during this nighttime break between the two halves of the sleep period, and some people even eat and drink during this period. It is incorporated into their regular lifestyle. The disruption of sleep architecture due to alcohol consumption often shows most prevalently during this middle period, when people who normally sleep the night through middle waking period wake.
"In his first sleep, call up your hardiest cheer," - Homer, Odyssey, Book 4, line 556