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The Spilsbury et al article1 looking at sleep behaviors in schoolchildren provides a welcome spotlight on a poorly understood area. A major criticism of the Spilsbury et al methods is that the study was not conducted for a significant length of time, just 7 days. Although the investigators attempt to reduce the effect of any confounders by removing data from children in the winter vacation, they do not in fact state which times of year the data were taken. Not including this information brings a significant degree of invalidity to the study since it is known that sleep duration can change with season,2 possibly via extraneous factors like temperature.3 A further possibility remains that variation in bedtimes measured in this study could be altered by cultural or social events since it is only 1 week that is measured. Spilsbury et al could have improved the quality of their study by conducting it over a greater number of weeks and stating the time of year the data were taken from, or even examining data from each individual at different times of the year.
The initial cohort had a higher proportion of nonminority children. To prevent any racial bias, an additional 243 minority children were included to bring the total study sample to 907. This may have conferred a selection bias onto the study as it does not appear to have been performed randomly and there is no indication as to how this extra sample was recruited. Spilsbury et al state that this addition to the sample was “balanced” with regard to their term or preterm status but do not indicate exactly what this means or how it was performed.
Sleep duration is measured by Spilsbury et al as a single variable, and the component parts of such a measurement seemed to have been overlooked. Sleep duration is measured in the study between “bedtime” and “waking up.” There appears to be no consideration or mention of sleep latency. Variations in sleep latency would have an enormous impact on the results, since simply because the children in the study state they spend more time in bed does not mean they are sleeping more. Sleep latency shows variation between individuals and can change in disease4 or with increased stress,5 neither of which are adequately taken into account in the study.
Correspondence: Mr Currie, Medical Teaching Centre, Warwick Medical School, University of Warwick, Gibbet Hill Rd, Coventry, CV4 7AL, United Kingdom (a.c.currie@warwick.ac.uk).
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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