The use of baby sleep bags has increased dramatically within the British population over the last decade. Parents report that sleep bags are a convenient alternative to traditional bedding, and help their babies sleep better by preventing them from kicking off blankets and becoming cold during the night. Such use is also supported by a study that suggested that their use is a preventative factor against SIDS (L’Hoir et al 1998) (possibly due to reduced potential for excess bedding and head covering, as might occur when traditional blankets, sheets or duvets are used), and by some advisory bodies including the Lullaby Trust. Very little epidemiological support exists for this, as few SIDS risk studies have examined sleep bag use. A recent UK case control study, found no association between sleep bag use and SIDS risk (Blair et al., 2009).
The relationship between bedding, temperature and SIDS
Since the 1980s there has been considerable scientific interest in the relationship between infant thermal environments and the risk of SIDS. A number of case-control studies conducted prior to the Back to Sleep campaign demonstrated that infants whose deaths were designated as SIDS tended to be exposed to greater insulation from more bedding (thicker or more layers), clothing, and higher ambient temperature than control infants. They also suggested the rate of SIDS was significantly higher during the winter months due to changes in thermal care practices during these months (e.g. Fleming et al 1990; Ponsonby et al 1992). Excessive bedding or high ambient room temperature and head covering may affect various aspects of infant physiology, with potentially negative consequences.
As bedding is a significant modifiable risk factor, understanding the way in which it is used is critical; previous research has found that parents have a tendency to use excessive bedding for infants (Fleming et al 1990), especially during the winter months according to variation in outside temperature, regardless of the ambient temperature within infants’ bedrooms (Ponsonby et al 1992). An important limitation of research into the safety and effects of all types of bedding, however, is the complexity of determining accurate absolute and comparative real-world insulation values of bedding. This is difficult for individual items, but is vastly more so when attempts are made to estimate the effects of multiple items or layers (Cronin-de-Chavez 2011).
Sleep bags, SIDS and safety
While infant sleep bags are now sold as a simple to use and safe alternative to sheets and blankets, it is important to recognise that little is known about two important factors: firstly, how exactly are sleep bags used? And secondly, how does the use of infant sleep bags affect infant sleep physiology? Are the claims made regarding sleep bag safety evidence based? One of the leading brands of infant sleep bag, ‘Grobag’, is endorsed by the Lullaby Trust, and these are explicitly marketed as being safer "We firmly believe that Baby Sleep Bags are a far safer form of bedding for babies than traditional blankets and sheets" (Grobag, 2010) despite there being very little substantial evidence to support this.
No research study has yet been published which a) establishes whether or not sleep bags are safer than traditional bedding with regard to SIDS risk, b) examines the effect of sleep bags on infant temperature during night-time sleep, or c) investigates how parents use sleep bags in combination with other bedding or clothing.
While a British Safety Standard for infant sleep bags exists, it is based on a relatively simplistic analysis of the tog value of materials used, and fails to take account of the many environmental and physiological variables that ultimately impact infant temperature (Cronin-de-Chavez 2011).
To date two studies have examined the physiological effects of sleep bag use, but have done so only during daytime naps, not during the night. One study (Sauseng et al., 2006) has demonstrated lower overall temperatures, and reduced temperature variance during sleep bag use compared to blankets. The usefulness of this study, however, is limited by the small sample studied (n=11), and reliance on thermographic imaging to ascertain skin surface temperature. A more recent paper by the same team (Sauseng et al., 2011) reported that there was no difference in body temperature of 15 babies after sleeping under blankets for 60 minutes, vs in a sleep bag for 60 minutes.
How parents use, and understand the purpose of, infant sleep bags is an important gap in current knowledge. How are infants dressed when using sleep bags? How do parents regulate infant’s temperature when using sleep bags? It has been suggested that one potentially problematic aspect of sleep bag use is parents’ inability to adjust bedding in response to changing environmental conditions (Arkell et al 2007). Does this lead parents to combine sleep bags with other bedding, or are parents who use ‘traditional bedding’ more likely to overwrap their infants at night due to lack of clear tog ratings on many infant bedding products?
A study examining the use of bedding among white British and South Asian mothers in the North of England (Cronin-de-Chavez 2011) found that 48% of the white British mothers used infant sleep bags in winter compared to 0% of South Asian mothers. Surprisingly, those using infant sleep bags had the highest levels of insulation compared to other forms of bedding. While this is ironic (as those mothers using the highest levels of insulation were also higher education, higher income, and potentially more SIDS aware, and infant sleep bags are often promoted as a simple and safe way to avoid over-wrapping and overheating), it does not necessarily indicate that these mothers were using an unsafe level of insulation.
Given the strength and ubiquity of recommendations made to parents regarding ambient room temperature and infant bedding, the environment in which infants sleep is a key area for further study, and one which is emphasised by the Lullaby Trust’s current literature: "There is a consensus view in the UK, not strongly evidence based, that an ambient room temperature of 16-20ºC, combined with light bedding or a lightweight well fitting sleeping bag, offers a comfortable and safe environment for sleeping babies but further research is necessary to establish this with confidence." (Lullaby Trust: http://www.lullabytrust.org.uk/document.doc?id=300).
Baby sleep bags are now a widely used alternative to traditional bedding, despite very little being known about the behavioural and physiological consequences of their use.