This post is about a study on childhood sleep duration and obesity in the British Medical Journal. There is alot of interest in determining whether sleep deprivation contributes to obesity in children and adults. Other studies have shown a link, and I have posted about it.
In this study, researchers took annual measurements of children as they aged from 3 to 7 years old. Sleep was measured at home with an actigraph, a form of home sleep study. The children wore the actigraph for 5 consecutive nights at ages 3, 4, and 5 years of age. Dietary intake was assessed over three days at ages 3, 4, and 5 years. It's not clear why sleep and dietary data were not collected for ages 6 and 7. Other data were collected about the children and their families.
The results showed that BMI tended to decrease slightly with age, and the average BMI was not elevated in the children. Average BMI's varied from 16.5 to 17.1, with overweight being defined as BMI 25-30. Data about the average fat mass index was not provided for unclear reasons. The sleep durations, as measured by the actigraph, were very similar across the three age ranges - 3 year olds averaged 11.1 hours per night, 4 year olds averaged 11.0 hours per night, and 5 year olds averaged 11.1 hours per night of sleep.
Stopping right there, and freely admitting I am not a statistician, these numbers seem pretty similar with regard to BMI and sleep duration. However, when the numbers were crunched by the researchers, they concluded that children who sleep less (at ages 3 - 5 years old) have a significantly higher risk of having a higher BMI at age seven, even after adjustments for other risk factors that have been implicated in regulation of body weight. In fact, each additional hour of sleep per night at ages 3 to 5 is associated with a reduction in BMI of 0.49 at age 7. The researchers and I agree that this is a pretty small effect when applied at the individual level. The researchers state that this small difference could become important for public health, when the data are applied to population levels. Also, the differences in BMI were more commonly due to an increase deposition of fat mass, rather than an accumulation of fat free mass (muscle or bone, eg).
This study did not examine the underlying reasons why shorter sleep durations might contribute to obesity. It could be due to hormonal, dietary, or behavioral factors.