Recent studies have cast new light on ‘infant sleep problems’. Instead of ‘sleeping through the night’ by 3 months as parents report, most infants continue to wake up in the night.26–28
The key issue is not why infants awake, since this is normal, but why around 25% of infants continue to ‘signal’ their parents upon waking rather than resettling autonomously like their peers.26–28
Against this background, this study found weak relationships between infants’ amount of crying at 5–6 weeks and the number of nights per week they woke and signalled parents at 12 weeks of age. This relationship was significant but meagre in the larger group studied, and non-significant in the other group. In both studies, the majority of infants who met criteria for prolonged crying or ‘colic’ at 5–6 weeks of age were settled in the night at 12 weeks of age, and ‘prolonged criers’ were not more, or less, likely than other infants to wake and signal parents in the night at 12 weeks. This finding suggests that the two sorts of problematic behaviour have different causes, rather than being due to a common underlying disturbance.
One proviso stems from evidence that parents report infant fuss/crying and night waking with signalling accurately, but miss occasions when infants wake in the night without attracting parental attention.16,26–28,46
The implication is that parental reports distinguish ‘biological’ sleep from quiet awakening poorly, highlighting the need for objective measurements for this purpose. However, the clinical phenomenon involves parental concerns and complaints to professionals and, for this purpose, parental reports are the method of choice. A second proviso is that just one outcome measure, the number of nights infants remained settled for 5 h or more without signalling parents at 12 weeks of age, was used to measure settled infant sleep-waking. This measure was chosen because of use in previous studies,33,34,39,40
while relationships between fuss/cry problems in the early months and sleeping problems at 8–24 months of age are similarly weak.29,30
The decision to target 12 weeks of age here reflects the evidence that most infants stop night waking and signalling by this age and retain this habit once developed, while infants who do not stop by 5 months of age are particularly likely to develop chronic sleeping problems.16,31
On the evidence here, this developmental sleep-waking ‘milestone’ is not predicted by prolonged ‘colicky’ crying 2 months earlier.
One reason for cautious generalisation is that studies have distinguished a small group, of around 5% of infants, who both cry a lot and have sleeping and other problems.47–49
However, unlike the infants assessed here, such cases are defined by: (1) prolonged crying which occurs beyond 4 months of age; (2) multiple areas of problematic behaviour, including crying, sleeping, feeding and/or other difficulties; (3) adverse long-term outcomes, including psychological and behavioural disturbances.47–49
Because they occur beyond the age when infant crying and sleeping behaviours undergo developmental transitions, and because of their much poorer outcomes, these cases may have generalised regulatory disturbances.7,50
Future research should benefit from distinguishing these different groups and developmental pathways, while health services may wish to prioritise cases with persistent, multiple, difficulties.
The importance of this study’s finding derives from the implication that, in most cases, prolonged crying in early infancy and sleeping problems after 12 weeks have distinct causes. This is consistent with evidence that infant sleeping problems usually involve a delay in inhibiting night-time signalling behaviours, so that most cases involve normal developmental processes and the factors which maintain or inhibit infant signalling at night. These factors probably involve the parenting environment. Four randomised controlled trials have found that ‘limit-setting’ parenting prevents continuation of night waking and signalling beyond 3 months of age.34,39,40,51
In contrast, comparative research has found that ‘infant-demand’ care reduces overall crying by a third at 5–6 weeks, but increases the number of infants who continue to wake and signal in the night at 12 weeks of age.35
That both methods of care have benefits, and limitations, helps to explain the controversy in the popular press about which is ‘better’. Guidelines have been published to help professionals to support parents in progressing from infant-demand to limit-setting care.11
Randomised controlled trials which evaluate this guidance should establish whether it generalises and is cost-effective.