Little sleep, big bones? FitBits reveal the link between poor sleep and weight gain

If you're a bit on the big side, there's a good chance you're also a bit on the tired side.

New research has confirmed the link between weight and sleep quality, but it remains unclear which causes which. 

"Inadequate sleep has been linked to negative health outcomes, including obesity, coronary artery disease, hypertension, type 2 diabetes, and even incident pneumonia," the study, published in journal JAMA Internal Medicine, said.

Previous research has mostly been done in the lab, with only small numbers of people for a few nights at most; or it's based on questionnaires. 

The latest study instead looked at data from more than 120,000 people - rather than getting them all into the lab for research or their memories, the scientists relied on up to two years of data from each provided by their FitBits - a wearable digital fitness tracker.

"The recent increased uptake of wearable sensors (also known as activity trackers) has made such longitudinal data increasingly obtainable with digital devices that offer unobtrusive monitoring of multiple health parameters, including sleep."

The results showed a link between the amount of sleep people had each night and body mass index - the less sleep, the more likely a person was to be overweight.

If sleep causes weight gain, the link appears to be particularly strong in women. Underweight women - those typically with a BMI lower than 18 - slept on average for more than seven hours a night. Women in the healthy weight range had a few minutes less, while it only took a drop of about half an hour's sleep to see the average BMI balloon out to a morbidly obese 45 .

Men got less sleep overall, and had to lose a lot more -  45 minutes - before seeing their BMI hit 45. 

Sleep vs weight.
Sleep vs weight. Photo credit: JAMA Internal Medicine

There was also a link between variability and weight - the less regular your sleeping patterns, the more likely you are to be overweight (and in some cases, underweight). Those in the healthy weight range have the most regular sleep. 

"We've known for a few decades now that sleep is definitely contributing to weight gain, however we don't know if it's a two-directional thing - it could be that weight gain is causing worse sleep," nutrition and sleep expert Geoff Kira of Massey University told The AM Show on Tuesday.

"You might have heard of a thing called sleep apnea, that impairs your sleep."

Sleep apnea happens when the affected person briefly stops breathing while asleep, and can be triggered by being overweight. The first clue you're suffering apnea might be waking up in the morning feeling like you haven't slept at all - it's been repeatedly waking you up all night to breathe, you just don't remember. 

"But it could also be that poor sleep affects the way that you eat, the way that you metabolise food," said Dr Kira.

"Your discipline to be able to control the foods you eat is going to be imparied with a lack of sleep. For some people that's really bad because they might have some kind of sleep problems - insomnia, or waking up during the night multiple times. That means that during the day they might get stressed more easily, and food is a way that people deal with their stress."

Whichever way the relationship goes, Dr Kira says getting more and better-quality sleep is important.

"The first thing to do is look at sleep hygiene - there is a lot of information online about it, and not very much of it is bad. The first thing to do is to ensure that your build-up to going to sleep is relaxing. 

"Unfortunately watching television in the hour before going to sleep is probably not the best way to go, purely because of the blue light from the screen - and also you might be a bit too excited... that gets your stress system up and you're not able to sleep. Try to relax, have a routine."