Fat babies sleep less

“Obesity risk for babies who sleep less,” is the headline in The Daily Telegraph today. The article says that “babies and toddlers who sleep for less than 12 hours a day are twice as likely to be overweight by the time they are three years old”. The research also shows that if this lack of sleep is combined with more than two hours of TV a day then that “increases the risk even further,” the newspaper says.

The story is based on a study that looked at the link between sleep duration and the weight of children at three years old. Parents were asked about their child’s sleeping and television watching habits, and medical records were used to determine the child’s weight and other measurements. It is possible that cultural practices or unmeasured characteristics of the families, such as tendencies to overfeed unsettled children or to provide TV sets in bedrooms, might partly account for the link seen in this study. The size of the effect shown here suggests that further study into the ‘sleep hygiene’ practices in infancy is needed.

Where did the story come from?

Dr Elsie Taveras from the Obesity Prevention Program and Center for Child Health Care Studies at Harvard Medical School and colleagues from elsewhere in the US carried out this research. The study was supported in part by grants from the National Institutes of Health and the Robert Wood Johnson Foundation. It was published in Archives of Pediatrics & Adolescent Medicine , a peer-reviewed medical journal.

What kind of scientific study was this?

This was a prospective cohort study in which the researchers wanted to test whether the link between duration of sleep and weight gain in older children also applied to younger infants and toddlers. It followed 950 children born to mothers who had previously been enrolled in a study of pregnancy and child health in Massachusetts. All the mothers enrolled had to be fluent in English and to have attended the final three-year clinic visit with their baby. Some were excluded if they had had twins or if insufficient height or weight data had been collected during the course of the study. Of the 2,128 women who potentially could have been enrolled, only 915 were included following this process. This resulted in a group where mothers were predominantly white and had been educated to college level, with slightly higher incomes than the original group.

Using postal questionnaires, the researchers calculated an average sleep duration for the babies at six months, one year and two years. At six months and three years, the mothers and their babies attended the clinic for measurements of child length, height and weight. At three years, the researchers also measured skin-fold thickness. Modelling and statistical analyses were used to assess the relationships between the characteristics collected by questionnaire and the measurements collected at clinic visits. The main factors of interest were body mass index (BMI) adjusted for age and sex (BMI z score), skin-fold thickness and weight (with overweight defined as being in the top 5% of BMI expected for three-year-olds of the same sex).

What were the results of the study?

Children slept, on average, 12.3 hours through the day. At age three years, 83 children (9%) were overweight. The mean BMI z score was 0.44 and the skin-fold thickness was 16.66 mm (the sum of measurements taken at two points: behind the arm and below the scapula). The researchers adjusted for several factors that they thought could influence weight, such as maternal education, income, the mother’s BMI before pregnancy, marital status, smoking history and breastfeeding duration. They also looked at the effect of the child’s race/ethnicity, birth weight, daily television viewing and daily participation in active play.

After statistical analysis, the researchers found that less than 12 hours a day of sleep was associated with a 16% higher BMI z score, a 0.79mm higher sum of skin-fold thickness and a doubling of the chance of being overweight. When the associations of sleep duration (less than 12 hours) with TV viewing (more than two hours) were modelled, the researchers found that these children had about a 17% higher chance of being overweight at three years. This suggests that at least part of the increase in rates of overweight children who have slept for less than 12 hours is due to an increase in TV viewing. On average, at two years old, the children had 1.4 hours of TV viewing a day and three hours a day of active play.

What interpretations did the researchers draw from these results?

The researchers say that “daily sleep duration of less than 12 hours during infancy appears to be a risk factor for overweight and adiposity in preschool-aged children”. They suggest that parents and their clinicians use strategies, including sleep hygiene techniques, to improve sleep duration among young children as these may be important in preventing children from becoming overweight.

What does the NHS Knowledge Service make of this study?

This is a well-designed, reliable study that used conventional statistical techniques to look at the associations or links between a number of childhood and infant characteristics and weight at three years. The authors acknowledge some strengths and limitations to the study, which in general do not affect the reliability of the conclusions.

• The design, a prospective cohort study, allowed data to be collected from age six months to three years. This is a strength because it allowed the researchers to examine the changes in risk factors over time. 
• Another strength was the large number of adjustments made for socioeconomic, demographic and environmental predictors of weight and obesity. The adjustments allowed the authors to reduce, as far as possible, the chance that these factors might influence the link between sleep through infancy and weight at three years.
• The researchers adjusted for all these known predictors. However, it is still possible that other unknown or unmeasured differences between the groups who slept more than 12 hours and the group who slept less than 12 hours could account for the link. Such differences could include the tendency for mothers to feed unsettled infants to calm them down.  
• How relevant the results of this study are for non-white ethnic or other socioeconomic groups has not been tested in this study. There may be cultural factors that determine whether children are encouraged to have TVs in their rooms at young ages which may not apply to other countries.

Designs such as randomised controlled trials will be required to test the theory that the ’sleep hygiene’ practices suggested but not described by these researchers, if followed through infancy, could lead to fewer overweight three year olds. Randomisation in such studies would also tend to balance the influences of any unknown or unmeasured factors.

Sir Muir Gray adds...

Obesity is primarily a behavioural and environmental problem; more exercise will increase the need for sleep and prevent obesity. Exercise is the missing link.