Is 'a solid 8 hours' normal human sleep?
Studies in present-day non-industrialised societies show that sleep cycles for both adults and children are governed by the daily cycles of day and night. Periods of darkness and daily activities provide a framework for regular periods of night-time sleep. Sleep also occurs during the day, however, and people drift into and out of sleep as needed, regularly waking from, and returning to, sleep during the night as well as in the day (Worthman and Melby 2002, Worthman 2008). Sleep time is not restricted, overall, and sleep itself takes place in a setting very unlike that of modern post-industrial society. Noise from other people and animals provide a background to sleep, and sleepers rest upon mats, skins, platforms, but there are no mattresses or pillows (Bower 1999). Researchers have suggested that sleep divided into segments - often proposed to be a period of around 8 hours divided by one or more periods of wakefulness - is characteristic of the sleeping patterns of human ancestors, and therefore represents an evolved norm for humans (Ekirch 2001).
In the industrial world the opportunity to sleep as needed gradually disappeared as a consequence of technological advances (the development of artificial light blurring the natural distinction between day and night, for example) and changes in working practices. Daytime activities became highly organised, regimented to fit in with national advances in transport (trains) and the resulting standardisation of 'time' itself. In the modern post-industrial period people - working or otherwise - are expected to function within a highly proscribed and defined schedule, and the constant and ubiquitous availability of personal entertainment such as TVs and the internet has further eroded the time available for sleep. Today's expectations of 'good' sleep - 8 uninterrupted hours in a quiet, dark environment - are therefore far removed from the biological norm which our bodies and minds have evolved to expect.
Going even further back into our evolutionary past we might consider whether the way we arrange our sleep environment affects our ability to sleep for prolonged periods. Although not specifically about infant sleep, this BBC Earth article about the differences in the sleep of apes (who, like us, sleep lying down on platforms they have constructed) and monkeys who sleep sitting up and maintaining vigilance, reports on some interesting research!
Changing expectations for infant sleep
Present-day UK sleeping arrangements -- sleeping alone or in pairs in warm, quiet, private rooms with spacious beds -- is a relatively recent historical development. Less than 200 years ago it was the norm, in UK and US households, for mothers and babies, and indeed for whole families, to sleep in close contact with each other. Around the world in non-Western cultures sleep is a social activity that people do in groups. Beds are basic, often on the floor, and pillows are thin or hard, or non-existent. Special rooms for bedrooms, special clothes for sleeping, and separate sleep locations for parents and children became popular as working people began to earn disposable income. As houses and living arrangements changed, so did our sleep behaviour, and our attitudes about infant sleep.
Nowadays Western parents often feel under pressure to help their babies develop independence at an early age. There is a common expectation that a 'good' baby sleeps through the night from an early age, sleeps alone, and does not require attention in the night. It is often a goal of parents to 'help' their baby accomplish this feat as quickly as possible. Based on what we know about infant evolutionary biology, expecting a human baby to sleep alone and for prolonged periods, seems rather unrealistic. The situation came about as a result of dramatic cultural and technological changes in Western society during the late 19th and early 20th centuries. These changes - notably industrialisation, the medicalisation of childbirth and the rise of 'scientific parenting' dramatically altered the ways in which adults and children lived, worked and slept. Some of these changes represented drastic yet untested alterations in baby-care practices, the effects of which are only just beginning to be understood. The mismatch in what today's parents might expect regarding infant sleep, and their baby's biological abilities regarding sleep, lead to some unnecessary conflicts.
Infant sleep around the world
Around the world infant sleep experiences can be very different from those of the majority of Western infants. Many studies have reported such differences:
- Mindell et al. (2010) reported that in Asian countries, infants (0-3 years) had a significantly shorter total sleep time than infants from Euro-American countries.
- Furthermore, young Asian infants were significantly more likely to room-in and co-sleep with their parents, whereas for Euro-American infants, rooming-in was only prevalent in the early postnatal period and decreased with increasing infant age.
- Customs such as mother-infant sleep separation are characterised as child neglect by Mayan parents, (Morelli et al. 1992) and 'unkind' by Italian parents (Wolf et al. 1996).
In many non-Western populations, babies and mothers sleep in continuous contact, regardless of time of day or night. Babies sleep on the mother's body or close to her, often in some form of carrying device during the day, and in her sleep space at night. Babies fall asleep while their mother works and/or completes her daily activities. These babies do not require silence or darkness to sleep, and they are not expected or 'trained' to conform to a schedule (Mead cited in Ball 2007, also Whiting 1981, Gantley 1993). Within Western societies before the late nineteenth century, these aspects of infant sleep were also the norm, as demonstrated by comments found in physicians' child-rearing guides for mothers (Hardyment 1983). The cultural aspects of sleep have therefore changed dramatically in our culture, but sleep biology has not changed.