How does sleep training work?
From what is known as a 'behaviourist' point of view, sleep training methods work by either rewarding or encouraging desirable behaviour (e.g. using bed-time routines to help babies learn it is sleep-time), or by not-rewarding undesirable behaviour (ignoring babies' when they cry in the night). 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 (also called Controlled comforting) fall into this category.
Although from a behaviourist perspective, we can see why and how these methods work (i.e. they reduce infant-related sleep disturbance for parents), there are alternative viewpoints that make the picture less clear.
- 'Extinction' methods require that both parents and babies break the link between crying and consistent parental response. This means breaking the link (responsive caregiving) that has evolved to ensure infant survival that has evolved to ensure infants' survival, which feels wrong to babies (hence they protest) and feels wrong to most parents (hence it feels traumatic to undertake).
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that is not crying is in fact asleep, or is in what is known as a 'dissociative' state (meaning that the baby has 'withdrawn' in response to the stress of being left alone with no care-giver response and has behaviourally / neurologically ‘shut down’). If the baby who has become quiet 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 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 parents and reduced waking (or crying) at night for babies (with the presumption of improved infant 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 for longer more quickly than is normal for their stage of development may put them at increased risk of SIDS. Unusually prolonged or deep sleep, from which a vulnerable baby cannot easily arouse, has been associated with increased SIDS-risk.
- Sleep training ultimately results in babies being left to sleep on their own. Indeed, being left alone to 'settle' is a central aspect of many sleep-training techniques. Research into SIDS risk factors has shown that when babies sleep alone while under 6-months of age they have a significantly increased chance of SIDS compared to those who sleep in the same room as a parent, for both daytime and night-time sleeps.
- 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 most feeds may shift to the day-time if none happen at night this can suppress maternal lactation physiology, causing milk supply to dwindle . In addition to the other effects of early weaning of infant health, early cessation of breastfeeding is also a factor that increases the risk of SIDS.li>
The unknown effect of separating mothers and babies
As illustrated in How Babies Sleep a human baby’s evolutionary biology requires them to be close to their mother, relying on her for food, warmth, and 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 dyad.
A recent study 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 of ‘controlled crying’ 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 been altered, 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.