We all know being overweight affects our health in many ways. It seems weight and sleep are also related.

A study reported in Science Daily (2006) found that women who slept five hours per night were 32% more likely to gain weight (33 pounds or more) and 15% more likely to become obese compared to women who slept seven hours per night. Women who slept for 6 hours were 12% more likely to have major weight gain and 6% more likely to become obese compared with women who slept 7 hours a night.

A total of 68,183 middle-aged women were followed for 16 years. They reported their typical sleep habits, and their weight every two years throughout the course of the study. When the study began, women who slept five hours or less per night weighed, on average, 5.4 pounds more than those who were sleeping seven hours, and over the next 10 years gained an additional 1.6 pounds more. That may not sound like much, but even a small difference in weight can increase a person’s risk of health problems such as diabetes and hypertension.

The participants’ diet and exercise habits were studied to see if they might account for the findings, at least in part. Earlier studies had indicated that after only a few days of sleep restriction, the body’s appetite controllers (hormones ghrelin and leptin) made people feel hungrier, so it was thought that those subjects who slept less might eat more, but in fact they ate less. That suggests that appetite and diet are not accounting for the weight gain in women who sleep less. The subjects were also asked about their level of physical activity, such as jogging or playing tennis, but no differences in exercise routines were found able to show why women who slept less weighed more.

Although the study provided no answers as to why less sleep might cause weight gain, there are some possibilities that deserve further study. It is thought that reduced sleep might bring about changes in the rate of calories burned while sleeping. Another culprit recently discovered is non-exercise associated thermogenesis, or NEAT, which in simple terms means people who sleep less toss and turn less, and so burn fewer calories while sleeping.

A newer study, conducted by Sarosh Motivala, Assistant Professor of Psychiatry at the Semel Institute for Neuroscience and Human Behavior at UCLA and colleagues, examined two hormones that are chiefly responsible for telling the body when it is hungry and when it is full, ghrelin and leptin. Ghrelin is a peptide secreted by the stomach. It stimulates the appetite, and its levels increase before meals. Leptin is secreted primarily by the fat cells and prompts the hypothalamus in regard to the degree of fat storage in the body. The amount of leptin signaling the hypothalamus indicates whether there is a calorie shortage – in which case, hunger occurs – or if the body has accumulated an overage of calories, in which case, energy expenditure is promoted.

Healthy sleepers were compared with insomniacs and researchers measured the levels of the two hormones at different times through the night. These measurements revealed that leptin levels stayed about the same between the two groups, while levels of ghrelin measured 30% lower in the insomniacs. Thus it would seem that lower levels of ghrelin would keep us thin – high levels stimulate appetite, after all.

But after comparing his findings with earlier studies on sleep deprivation, he thought perhaps a switch might occur during the day. People who suffered from insomnia appeared to have a abnormal regulation in energy balance that might lie behind the weight gain among insomniacs. This abnormal regulation amounts to a double whammy of increased ghrelin and decreased leptin to stimulate appetite beyond the norm.