People who sleep less than seven hours a day are 30% more likely to be obese than those who get nine hours or more of sleep.

Update 2015-12-14

First email:

"Dear Ben,

Thank you for your email to NHS Choices website requesting evidence to support the claim that sleep deprivation can lead to weight gain. I refer you to these two articles in the first instance.
http://www.ncbi.nlm.nih.gov/pubmed/18239586

http://www.hsph.harvard.edu/obesity-prevention-source/obesity-causes/sleep-and-obesity/
Kind Regards,
The NHS Choices Service Desk"
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I could find no data in these articles that supported the claim on the website, so I asked precisely where the data were and received a second email:

"Dear Ben

Thank you for your email

The Nurses’ Health Study where short sleepers had 30 percent higher risk of gaining 30 pounds over the course of the 16-year study can be found at (http://www.ncbi.nlm.nih.gov/pubmed/16914506?dopt=Citation)2006; 164:947-54.

If you have any further questions feel free to contact us.

Kind Regards,
The NHS Choices Service Desk"
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I read the study and found multiple problems with the way the data were presented on the NHS website, which I detailed in my reply:

"Hi,

Thank you again for your response. There are several problems that I would like to highlight with the way these data are presented on the NHS website.

1) The study you refer to uses 7 hours of sleep as a baseline, not 9 hours.

2) The data from Table 3 do not support the claim. Using data from model 3 (adjusted for the most confounding factors):

- Women sleeping 5 hours or less were 28% more likely to gain 15kg over the study period and 15% more likely to become obese than women who slept 7 hours.
- Women sleeping 6 hours were 10% more likely to gain 15kg over the study period and 6% more likely to become obese than women who slept 7 hours.

There are two problems here. First, the NHS website only uses the figure (~30%) from the most extreme case for a 15kg weight gain (5 hours or less sleep), but the weight gain for sleeping 6 hours is much lower. Hence, the 30% figure is not representative of those who get 6 hours sleep. Second, a 15kg weight gain does not necessarily lead to obesity (e.g. some of those individuals were already obese). The study specifically states different figures for women who became obese, which are 6-15%.

3) The way in which this is worded doesn't paint a very clear picture of causality. 'People who sleep less are more likely to be obese' doesn't mean that sleeping less causes obesity. Perhaps obesity causes people to sleep less, or there is a confounding factor. The longitudinal data from the paper support the argument that sleep deprivation leads to weight gain, but this isn't presented clearly.

4) This is only one study and is limited to middle aged women, so attaching blanket statistics without context can be misleading. The literature does support the theory that sleep deprivation increases the risk of subsequent weight gain and obesity, but there is no magic number to attach to this.

Might I suggest the following wording to clear up any ambiguities:

"Sleeping less may lead to weight gain and obesity! Several studies have found that people who sleep less than 7 hours a day tend to gain more weight and have a higher risk of becoming obese than those who get 7 hours of sleep."

Please let me know your thoughts on this.

Best wishes,
Ben"
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To which I received the following reply:

"Dear Ben

We have made the editorial amendments suggested, though it may take a day or two to show up on the site while the page refreshes.

If you have any further questions feel free to contact us.

Kind Regards,
The NHS Choices Service Desk"
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The website has now been updated to the following:

"Sleeping less may mean you put on weight! Studies have shown that people who sleep less than seven hours a day tend to gain more weight and have a higher risk of becoming obese than those who get seven hours of slumber."

Not perfect, but an improvement!

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