Where have our ideas about 'normal' infant sleep, and 'sleeping through the night', come from?
Guidelines relating to what is 'normal' for infant sleep, along with popular perceptions of normal or 'problem' infant sleep behaviour, have largely been based upon the research of Moore and Ucko (1957). based on their study of approximately 160 infants they reported that 70% began sleeping through the night at the age of 3 months. This finding subsequently became codified in paediatric texts and guides for parents as 'infants should be sleeping through the night from 3 months' (e.g. Better Homes & Gardens Baby Book 1965). However this interpretation was misleading on at least 3 counts:
- 'Sleeping through the night' was defined as a 5 hour stretch from midnight to 5am, during which the parents were not woken by the baby's crying or fussing. However prevailing infant care fashions at the time meant that parents were putting their infants to sleep in a separate room, so although parents may not have been woken by their infant, it is likely that infant wake time was underestimated (Anders 1979).
- Although at 3 months 70% of infants began experiencing 'sleeping through', 30% did not do so until later. Additionally, although sleeping through may have been experienced by 3 months, the researchers did not report the babies were regularly sleeping through the night. Reverting back to frequent night waking was common - Moore & Ucko report that half of their sample reverted back to night waking regardless of the age at which parents believed they had begun 'sleeping through'.
Moore & Ucko's study was done at a time when infants were predominantly fed cow's milk based formula, and supplemented early with 'baby rice' or 'infant cereal'; we now know that what an infant is fed affects both their sleep patterns and the frequency of parent-infant interactions during the night.
Recent research by Henderson et al (2010) investigated infant night-time sleep development from birth to 12 months against 3 criteria:
Criterion 1: sleeping uninterrupted from 24:00 to 05:00 hours (from Moore and Ucko); Criterion 2: the 8-hour criterion, sleeping uninterrupted for 8 hours minimum between sleep onset and time awake in the morning; Criterion 3: the family-congruent criterion, sleeping uninterrupted from 22:00 to 06:00 hours." (p1083) Infants were judged as having met the criterion of having 'slept through' if they had done so for 5 of 6 reported nights in one week, or on 80% of occasions if there were missing data.
Henderson et al found that at 3 months, 58% of infants met criterion 1, at 4 months 58% met criterion 2, and at 5 months 53% met criterion 3; and concluded that criterion 3 should be used by clinicians to define 'sleeping through the night' for infants from 4 months old as it is "met by 50% of infants at 5 months of age, is congruent with family sleep patterns, and falls within typical sleep times" (p1086).
However, by the time they reached 12 months of age, 27% of infants still did not meet criterion 3. Despite this, Henderson et al recommend that "interventions intended to prevent infant sleep difficulties" be implemented in the first 3 months of life.
A later study by Henderson et al. (2013) conducted a community survey in New Zealand of parents’ (children under age of 2) opinions about and expectation of infant sleep. They found that mothers disagreed with Moore and Ucko’s criterion 1 as a definition of ‘sleeping through the night’. An ideal nocturnal sleep duration was considered to be at least 9.6 hours, occurring between 20:00 and 06:30 hours, which was close to the definition of ‘sleeping through’ (criterion 3) advocated in Henderson et al’s 2010 research. However, it should be noted that parents were not asked if this ideal matched their lived experiences. Almost half of the mothers reported they had sought advice on their infants’ sleep.
Although Henderson et al have made some contribution to defining a socially relevant meaning of 'sleeping through the night', the relevance of their inferences about the development of normal sleep consolidation - on any of their three criteria - is limited by lack of data on either infant sleep location or infant feeding method, both of which are known to have a profound impact on real and perceived sleep duration. See here for more information on this.
Other research looking at ‘normal’ sleep in infants and children.
Galland et al. (2012) carried out a systematic review of the scientific literature with the purpose to provide an international average reference point for normal sleep patterns in infants and children (0–12 years). They found that sleep patterns show developmental trends for sleep duration (decrease 0-12 years), number of night wakings (decrease from 0 to 2 years), longest sleep period (increase from 0 to 2 years), and number of daytime naps (decreasing up to age 2). The findings show a wide variation in what is considered normal sleep patterns, particularly in the very young. Variation was also seen between Asian countries and non-Asian countries suggesting socio-cultural differences. These are important consideration when dealing with perceived infant/child sleep problems. There were, however, limitations; data was parentally self-reported and infant feeding and sleeping location was not available within the studies reviewed. As other studies have suggested (such as Yoshida et al. 2015 and St James-Roberts et al 2015), different data collection methods can provide fuller and sometimes different account of infant sleep behaviour.
Longitudinal studies are another important method of gathering data as they allow researchers to track the same people over an extended period of time. A longitudinal study (Price et al. 2014) looking at children’s sleep patterns from 0 to 9 years, like Galland et al (2012), found a striking variation in sleep duration at all ages, in sleep onset time and, especially, wake times. The findings showed that by primary school age sleep onset and waking times were markedly later on weekend days, a distinction that was absent from the data analysed by Galland and colleagues.