A century or two ago, people used to sleep twice a night. In the evening, they would change into their pajamas, put on a nightcap and snuff out the candle. They would sleep for several hours and then they would get up in the middle of the night, light the candle and engage in some quiet activities in the next two or three hours. They would read books, write, smoke, pray, converse… When they were done with their night activities, they would go back to bed for another spell of sleeping until dawn. Together with the intermission, their night lasted around twelve hours.
Roger Ekirch, a history professor at Virginia Tech, arrived at this discovery by analyzing numerous records, such as court documents, diaries, books, notebooks, letters… He found more than 500 textual references confirming the habit. The most common terms used in the literary sources from the time are “first sleep” and “second sleep”. Segmented sleep is mentioned from Homer’s Odissey to the modern writings of a Nigerian tribe. According to Ekirch, the habit disappeared with the advent of street and house lighting.
As it turns out, however, the notion of two sleeps is not that unusual and unnatural, and is also grounded in science. Psychiatrist Thomas Wehr of the American National Institute of Mental Health conducted a study on photoperiodicity, investigating how the length of day affects the period and length of sleep, hormone levels and human behavior. In 1992, he published his findings in the article, “In Short Photoperiods, Human Sleep is Biphasic”, in the Journal of Sleep Research.
Wehr’s study, which lasted four weeks, involved fifteen young men from the vicinity of Washington, D.C., who were deprived of artificial light, much as they would have been in a time without electric lights. They were transferred from their usual 20th-century routine, where one is awake for sixteen hours a day, to an environment with only ten hours of daylight, which is common in winter. Every evening during the four weeks, the subjects came to the laboratory and spent fourteen hours in darkness, sleeping and relaxing as much as they could.
A very interesting phenomenon occurred. At first, the subjects slept for a very long time, making up for the sleep they missed in their hectic lifestyles. However, when they got used to the new conditions, their sleep started following a rather interesting pattern: they slept only one hour longer than before, but their sleep was segmented within a twelve-hour period. First they fell into a deep sleep that lasted four or five hours and then slept as long before dawn, but now predominantly in the REM phase.
Between the two bouts of sleep, there was a waking interval of about one or two hours. They were totally awake, tranquil and relaxed during the interval, and their cerebral activities were similar to those during meditation. All this time, their levels of the hormone prolactin, which are, incidentally, twice as high in the period of sleep than in the wakeful state, were measured. This high level of prolactin in the wakeful state explains why men were surprisingly tranquil during the waking interval. It is known that in women the hormone is responsible for stimulating the production of milk after childbirth, and it also induces animals to rest and birds to brood.
Even though it seems that it is in human nature to sleep in two parts during the night, it must not be forgotten that this changes in the summer months, when nights are shorter and when sleeping in two phases is nigh on impossible.
(M.Đ.)