Too little sleep won't make you fat

Skimping on sleep is unhealthy but it doesn't make people fat.

Chronic sleep deprivation is thought to be a risk factor for weight gain. While several studies have linked higher body mass index (BMI) to shorter nightly sleep, most have been cross-sectional, meaning they looked only at a single point in time - making it hard to prove whether sleeping too little leads to weight gain or vice versa.

Also, most studies have relied on people's own estimations of how much they sleep at night, and it is well known that self-reports may be inaccurate and biased. 

To address this problem, researchers in America had people wear a wrist activity monitor,a watch-like movement-tracking device called an actigraph, which can measure sleep duration as well as sleep fragmentation, or how often a person wakes up during the night. They studied 612 people participating in a long-running study of heart disease risk, all of whom were in their 40s.

While shorter sleep duration was indeed associated with higher BMI, as was more fragmented sleep, adjusting for ethnicity and socioeconomic status weakened the relationship. And people who slept less at the study's outset were no more likely to gain weight during the five-year follow-up period.

The researchers also found that the link between shorter sleep times and higher BMI was much weaker for people who did not snore, and much stronger for those who did.

Sleep apnoea, in which a person wakes up gasping for breath several times during the night, has been linked to obesity, but snoring is not a very accurate marker of apnoea. While most people with sleep apnoea snore, not everyone who snores has sleep apnoea. Snoring could be a particularly good indicator of apnoea for heavier people, which might explain the findings.

The findings need to be confirmed in other groups of people and it would also be helpful to use a good clinical diagnosis of apnoea to separate out those with a real sleep disorder from people who are simply snorers. Research should now delve more deeply into the type of sleep people are getting, and whether this is influenced by BMI or vice versa, the researchers concluded.