Erasmus's book of adages or sayings published in 1500 included the following: "In utramvis dormire Aurem." This is usually translated into English as "He may sleep on either ear". Being able to sleep on either side (and thus on either ear) suggested ease, comfort and a sound sleep. The saying also suggests the long legacy of advice about sleep and rest.
As I stare at the blue light of my laptop screen to type this I am, according to recent research, probably interfering with my ability to sleep on either ear later tonight. “Sleep hygiene” is no longer a topic limited to exhausted parents hoping to coax children into routines. We are inundated with advice about the importance of a good night’s sleep and how to get it.
But what if a good night’s sleep hasn’t always been defined the same way, as a block of seven to eight hours of uninterrupted sleep taking place during the night?
Historian A Roger Ekirch’s studies of the history of sleep in the pre-Industrial world suggest that people commonly experienced “segmented” or “bi-modal” sleep. In fact, waking in the middle of the night was considered normal. Writers and diarists, medical practitioners and lay people referred to first and second sleep periods.
The first sleep often ended around midnight or later. Upon waking after a first sleep, the early modern European was not necessarily perturbed. They might lie awake, go to the toilet, have sex, write down ideas, eat, smoke, or get up to do a few tasks. After this (relatively) short interruption they often returned to sleep for several more hours.
There is scientific and anthropological evidence to support the idea that segmented sleep is a natural condition rather than always evidence of insomnia. In a study published in the early 1990s by Dr Thomas Wehr, human subjects who lived without artificial light began to exhibit a sleep pattern much more like that described by Ekirch's evidence.
They took time to fall asleep, slept for three to four hours and then woke. After a period of wakefulness they returned to sleep for around four more hours. Anthropological accounts of sleeping and waking patterns of nonindustrial societies in the 20th century have often observed similar periods of wakefulness in the middle of the night.
Blue light from screens might be the most recent concern of scientists trying to help us sleep better, but light of all kinds may be interfering with our sleep. What the objects of study in research by historians, doctors and anthropologists shared was an absence of light sources after sunset.
Of course, fires of various kinds from candles to gas lamps were often available. Yet none of these lights has the strength of the electric lamps we are surrounded by. It is really only in the past 200 years that we are able to almost replicate daylight after dark. All this extra light seems to be changing how we sleep.
Our sleep patterns are probably determined by cultural ideas as well as by environmental conditions. The idea of “business hours” shapes our expectation of uninterrupted night-time sleep perhaps as much as our biology. A more time-conscious industrial and post-industrial society must have workers who begin work at 9am, rested and ready for the day.
Changing expectations of living conditions, and government regulation, have reduced night-time noise even for people living in cities. We complain if neighbours blast their music during sleeping hours; city councils limit evening “noise pollution”. We worry lest our crying babies might wake the neighbours and we rarely sleep five or 10 persons to a room. We have created the ideal social conditions for getting the type of sleep we now consider the ideal.
None of this evidence contradicts the idea that sufficient sleep is important for health. Indeed, centuries of advice on sleep found in books of health, medical advice manuals and popular sayings only underscore the persistence of interest in getting a good sleep. Limiting exposure to light sources after dark seems sensible.
However, we also have to consider that the definition of “a good sleep” is not the same at every time, in every place and for every person. Next time you find yourself waking in the middle of the night consider yourself in good historical company. And maybe try to sleep on your other ear.
Dr Juliana Adelman lectures in history at Dublin City University