Friday May 27 2011
Lack of sleep may not have caused children to be overweight
“Children who get insufficient sleep at night are more likely to become overweight,” reported BBC News.
The news was based on a study of 244 children, whose sleep patterns were assessed between the ages of three and five years old to see if they affected their body mass index (BMI) at the age of seven. The study found that, on average, children who slept for an hour less in their earlier years had a later BMI that was about 0.4 points higher. The study has some strengths, such as its use of objective measurements of sleep, but is limited by its small size. It is also difficult to be sure that sleep directly caused the differences seen in BMI.
It is clearly important that children get enough sleep, but it is not possible to say from the results of this study alone that interventions to increase children’s sleep will reduce their risk of being overweight. At present, the best advice to prevent a child becoming overweight is to make sure that they do enough physical activity and eat a healthy, balanced diet with the right amount of calories and nutrients for their age group.
Where did the story come from?
The study was carried out by researchers from the University of Otago in New Zealand, who also funded the study along with the Child Health Research Foundation, the New Zealand Heart Foundation, and the Dean’s Bequest-AAW Jones Trust. The study was published in the peer-reviewed British Medical Journal.
BBC News, the Daily Mail and The Daily Telegraph covered this story. While they accurately described the study, they did not highlight its limitations.
What kind of research was this?
This prospective cohort study investigated whether the duration of children’s sleep between the ages of three and five was related to their body composition and risk of being overweight at the age of seven.
The researchers say that previous studies have shown a “relatively consistent” link between shorter sleep duration and an increased risk of children being overweight, but that most of these studies had limitations. For example, they were mainly cross-sectional studies, which cannot establish whether the shorter sleep duration preceded the child being overweight. The studies that did follow children up over time relied on parents to report how long their children slept for, rather than measuring this objectively.
The current study aimed to do better than these studies by following children up over time to ensure that their sleep duration was measured before they were overweight, and by using an objective measure of sleep duration. A prospective cohort study is the best type of study for investigating this subject. However, children who sleep less may have other habits that could contribute to them being overweight, which need to be taken into account in the study’s analysis. The researchers addressed many of these habits.
What did the research involve?
The researchers enrolled 244 children who were part of a birth cohort, a study that follows all of the children born in a specific period and location. They followed these children up and looked at whether their sleep patterns between the ages of three and five years were related to their body composition and body mass index (BMI) at the age of seven.
The children were recruited in Dunedin, New Zealand, at the age of three, and all children born at the Queen Mary Maternity Unit in Dunedin between July 19 2001 and January 19 2002 were eligible. The researchers excluded any children who were born prematurely, were one of a multiple birth (i.e. twins or triplets), were born with major abnormalities or whose mother had a severe illness after their birth. Of the 413 children who were eligible, 244 participated (a response rate of 59%).
The children attended the research clinic every six months between the ages of three and seven. Their body composition, height and weight were measured every year. Their dietary patterns, physical activity and sleep patterns were assessed at the ages of three, four and five. Parents filled in questionnaires about their children’s diets and how much TV they watched. The children’s levels of physical activity and sleep duration were measured using a motion-sensing monitor (called an accelerometer), which was worn around the waist. The monitors were worn constantly for five consecutive days. The parents also recorded when the children went to bed, went to sleep and got up each day over the same period.
The researchers then analysed whether the children’s average sleep patterns between the ages of three and five were linked to their body composition or risk of being overweight at age seven. The researchers took into account some factors that could influence the results (confounding factors), including age, gender, dietary habits, TV watching, physical activity, children’s BMI at age three and their mothers’ BMI, education, income, birth weight, ethnicity and whether they smoked in pregnancy.
What were the basic results?
Most of the children who enrolled (83%) were successfully followed up to age seven. Between the ages of three and five, average sleep duration was about 11 hours a day. At age seven, the children’s average weight was 25kg and their average BMI was 16.7. At this age, 28% of girls and 22% of boys were classed as overweight (defined as having a BMI in the highest 15% expected for their age group).
The researchers found that children who slept longer between the ages of three and five had lower BMIs and were less likely to be overweight at age seven. Once the researchers took into account all the factors they thought might influence the results, including BMI at age three:
- Each additional hour of sleep at ages three to five was associated with a reduction in BMI at age seven of 0.39kg/m2 (95% confidence interval [CI] 0.06 to 0.72).
- Each additional hour of sleep was associated with a reduction in the risk of being overweight of 56% (relative risk 0.44, 95% CI 0.29 to 0.67).
The researchers found that this difference was mainly due to difference in fat mass rather than non-fat mass.
How did the researchers interpret the results?
The researchers concluded that “young children who do not get enough sleep are at increased risk of becoming overweight,” even after they adjusted their results to account for the children’s initial weight and other factors that could have had an effect.
This study suggests that getting less sleep between the ages of three and five is associated with a greater risk of being overweight at age seven. The strengths of the study are its cohort design, use of an objective measure of sleep, and high follow-up rate. The study also had some limitations:
- The use of objective measures of sleep helps ensure that these measurements are more accurate. However, there may still be some inaccuracy with the measure used because sleep duration was based on motion, but children may lie still without being asleep.
- The researchers measured sleep, physical activity and diet intermittently throughout the study. Although this is better than many studies that only assess such measures once, these periodic measurements may still not have fully captured the children’s habits over the whole period. In addition, the researchers had to rely on the parents’ reports of their children's diet, which may have led to inaccuracy if, for example, parents were too embarrassed to report their children's diet accurately because they felt they ate too much unhealthy food.
- The researchers took several confounding factors into account in their analyses, but it is possible that these adjustments did not completely remove the effect of these factors. Other factors may also have had an effect, such as the fathers’ socioeconomic status, which was not taken into account.
- The study was relatively small, and only about 60% of those who were asked to participate did. This may mean that the results are not representative of all children and are more susceptible to being influenced by chance.
- The size of the effect on BMI was relatively small. The researchers say that while this might seem minor in individual children, the benefits for public health, if looked in the population as a whole, could be considerable. To help interpret the importance of the results, it would have been useful to see figures showing the BMIs and proportion of children who were overweight among the groups with different sleep durations at ages three to five, but these were not shown in the paper.
Based on this study alone, it is not possible to say whether lack of sleep directly caused the children to become overweight. Proving that one factor causes another requires the accumulation of a range of evidence, which will require more research in this area. Clearly, it is important that children get enough sleep, but it is not possible to say for certain whether this will reduce their risk of being overweight.
At present, the best advice that can be given to prevent a child becoming overweight is to make sure that they do enough physical activity and eat a healthy, balanced diet with an appropriate amount of calories and nutrients for their age group.