Getting one extra hour of sleep each night might shave a third of an inch off your waist and a couple of pounds off the number on the bathroom scale, a recent study suggests.

Among some 1,600 people in the UK, researchers found that those who slept more than eight hours a night had lower body mass index (BMI) - a measure of weight relative to height - and slightly smaller waists when compared to people who slept less than seven hours.

Longer sleepers also had slightly higher levels of HDL “good” cholesterol.

“Most of the findings are in line with what experimental sleep loss studies have shown. I think maybe a plus of this is that obviously it's a much larger sample than something you would see in a laboratory,” Namni Goel, a sleep researcher at the Perelman School of Medicine at the University of Pennsylvania in Philadelphia who wasn’t involved in the study.

For the study, published in PLoS One, Gregory Potter of the School of Medicine at the University of Leeds in West Yorkshire, and his colleagues analyzed four years’ of data from a national diet and nutrition survey that also tracks other health and lifestyle habits among people in England, Northern Ireland, Scotland and Wales.

The available data included self-reported sleep records and food diaries for 1,615 adults, along with height and weight and blood pressure readings. In addition, about half of the participants agreed to provide blood samples so the study team could examine various measures of metabolic health such as cholesterol, blood sugar and thyroid hormone levels.

Researchers divided participants into three groups based on their average sleep duration. The bottom third had an average of 5.88 hours of sleep per night, with a range of plus or minus 52 minutes. The middle third had an average of 7.26 hours of sleep per night, plus or minus about 15 minutes, and the top third got an average of 8.44 hours of sleep at night, plus or minus 40 minutes.

The study team found that people in the top third for sleep duration had BMIs that were about two points lower – the equivalent of roughly 7 pounds (3.2 kilograms) – compared to people in the lowest third.

The longest sleepers also had waist circumferences averaging 1.6 inches (4 centimetres) smaller than the shortest sleepers.

Each extra hour of sleep was tied to a third of an inch (0.9 cm) difference in waist size and 0.46 of a BMI point, the study team reports.

There were slight improvements with more sleep in some blood markers, but after researchers adjusted for other factors, the differences were not statistically significant, meaning they could have been due to chance.

The study team also didn’t find any association between sleep duration and diet or calorie intake, although Goel noted that food and sleep diaries are often inaccurate.

There are currently ongoing studies trying to figure out the mechanisms behind sleep and weight issues, said Goel, but she thinks it’s clear the source of the weight gain is overeating.

“Some of it is that when people are sleep deprived, they tend to go for high calorie, fatty, good tasting foods, and one of the ideas behind that is that it may be that some of the reward centres in the brain are affected by sleep loss. Those centres stimulate people to go for those higher fat foods,” she said by email.

Dr. Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis, noted that short experimental studies in research units show that short sleep increases hunger, increases appetite and increases food intake.

Klein, who wasn’t involved in the study, said people can get better sleep by focusing on a combination of two things: getting more sleep for the hours spent in bed and increasing the time they allocate for sleeping.

"Things like keeping the room where you sleep cool and dark, don't have a lot of lights on before bedtime, keep it dark, don't watch TV in bed," he said by email.

“Go to bed earlier, wake up later. Those are things that can be done by really making sleep a priority,” Klein added.