How does sleep training work?
From what is known as a 'behavioural' point of view, sleep training methods work by either rewarding or encouraging desirable behaviour (e.g. using bed-time routines to help babies realise it is sleep-time), or by not-rewarding undesirable behaviour (ignoring babies' crying). Since a wakeful baby who does not disturb their parents is not usually considered to be a problem, it is no surprise that many of the most 'effective' sleep training methods have focused on managing babies' crying, rather than their sleep per se. 'Extinction' methods - including Cry It Out and Controlled Crying - fall into this category, and are the methods that are supported by the research literature as being effective.
Whilst from a behavioural perspective, we can see why and how these methods work, there are alternative viewpoints, from which the picture is not so clear.
- 'Extinction' methods require that both parents and babies break the link between crying and consistent parental response. This means severing a link that has evolved to ensure infants' survival.
Almost no research has looked at the processes occurring in babies' brains and bodies during sleep training; we therefore have no way of knowing if a baby or child that is not crying is in fact asleep, or is in what is known as a 'dissociative' state (meaning that the baby has 'withdrawn' in response to the stress caused by being left alone, with cries not being responded to, and has shut off their normal responses to being alone i.e. crying). If the baby who has settled following sleep training is indeed asleep, is their sleep 'normal' or is it different to that of babies who have not undergone the process? Research in this area has provided a general lack of information regarding the long-term effectiveness and consequences of sleep training interventions.
Although we know very little about the physical and emotional effects of sleep training, there are some potential consequences that should be considered.
The effect of sleep training on SIDS-risk, and on breastfeeding
The goal of most sleep training is increased sleep for babies (and their parents) and reduced waking (or crying) at night.
Given that many sleep training methods appear to have some impact on the way babies sleep (or at the very least, on the way parents feel about their babies' sleep), it is important to recognise two potentially important consequences of artificially hastening babies sleep development processes.
- Encouraging young babies, especially those less than 6 months of age, to sleep longer or more deeply than is normal for their stage of development may put them at increased risk of SIDS. Unusually deep sleep, from which a vulnerable baby cannot easily arouse, is thought to be associated with SIDS. It also seems likely that 'success' in sleep training will result in babies being left alone to sleep. Indeed, being left alone to 'settle' is a central aspect of many sleep-training techniques. Again, research has shown that babies who sleep alone are at increased risk of SIDS compared to those who sleep in the same room as a parent, including for naps.
- Sleeping for longer at night reduces opportunities for breastfeeding. Some sleep training regimes emphasise that feeding and sleeping should be separated. Night-time breastfeeds are important for both initiation and maintenance of breastfeeding, as they have a much greater effect on the hormones needed to support breast milk production. While feeds may shift to the day-time if they do not happen at night, this may not be adequate to offset the damaging effects on the mother's breastfeeding physiology of not promoting milk removal and production at night, particularly if mothers return to work during the day.
The unknown effect of separating mothers and babies
As illustrated in How babies sleep, human babies have evolved to be kept close to their mother, relying on her for food, warmth, protection - all elements essential to their survival. The closeness of mother and baby is promoted by a link that binds them together - the bond of attachment - and some researchers propose that this bond is maintained by mothers and babies 'creating' and responding to 'cues' - crying, visual contact, touch.
Recent research suggests that not only does this link between mother and baby have practical aspects (baby cries, mother feeds or cuddles baby), it can also be observed in terms of physical synchrony of mothers and their babies. When the mechanisms that maintain synchrony are disrupted, the effects can be seen and measured as a biological imbalance in the mother-infant link. A recent study demonstrated that mothers and babies undergoing a controlled-crying intervention started out with matching, synchronous, hormonal stress responses (babies cried at bed-time, their stress hormone levels increased, and their mothers' stress hormone levels also increased). After three days, the babies had ceased to cry at night, and mothers' stress hormone levels dropped, however babies' levels - despite the fact they were no longer crying - remained high. This suggests that the babies' behaviour had adapted to being alone for sleep, but that their physiology had not. The response of the babies in this study lends support to the theory that babies who undergo sleep training via extinction may be learning to 'give up' rather than to 'settle' -- outwardly the two behaviours appear the same, but inwardly the babies physiology is very different. As well as being physically separated from their mothers, the sleep trained babies were no longer in physical synchrony with their mothers as their mutual stress response link (maintained by infant crying) had been broken.
Research in this field is still in its infancy; however, behavioural synchrony between mothers and babies has been well studied, and is associated with the maintenance of positive mother-infant relationships and normal infant development. Physiological synchrony between mothers and infants may not just exist to ensure babies get fed, and are not abandoned accidentally; it may also play an important role in maintaining mother-infant relationships, and in promoting normal emotional, social, and physiological development.
For more detailed information on all the issues related to sleep training see our Health Practitioners pages.