Updated December 15, 2014.
If you want to lose weight, experts say you need to get enough sleep. Specifically, researchers have reported that women who sleep 5 hours or less per night generally weigh more than women who sleep 7 hours per night.
These findings, presented at the 2006 American Thoracic Society International Conference, showed that women who slept 5 hours per night were 32% more likely to experience major weight gain (an increase of 33 pounds or more) and 15% more likely to become obese over the course of the 16-year study, compared to those who slept 7 hours a night.

Those women who slept 6 hours per night were still 12% more likely to experience major weight gain, and 6% more likely to become obese, compared to women who slept 7 hours a night.

This is the largest study to track the effects of sleep habits on weight gain over time; it included nearly 70,000 middle-aged women.

The women were first monitored in 1986, and they reported their weight every 2 years for 16 years. At the start of the study, the women who slept 5 hours or less per night weighed an average of 5.4 pounds more than those sleeping 7 hours. They also gained an additional 1.6 pounds more over the next 10 years. While that doesn't sound like a significant amount, it adds up. That's 16 pounds in 10 years, and 32 pounds over a 20-year period.

"That's an average amount -- some women gained much more than that," says lead researcher Sanjay Patel, M.D., Assistant Professor of Medicine at Case Western Reserve University in Cleveland, OH. "Even a small difference in weight can increase a person's risk of health problems, such as diabetes and hypertension."

Women Who Sleep Less, Eat Less...and Still Gain

The researchers looked at exercise habits to determine if they could account, in part, for the findings. But they didn't discover any differences in exercise levels or physical activity that would explain why the women who slept less weighed more.
Were the women who were getting less sleep also eating more?

The answer was no. In fact, the opposite was true.

"Prior studies have shown that after just a few days of sleep restriction, the hormones that control appetite cause people to become hungrier, so we thought that women who slept less might eat more," Patel says. "But, in fact, they ate less. That suggests that appetite and diet are not accounting for the weight gain in women who sleep less."

Understanding the Reasons

The study participants did not identify the specific factors that contribute to weight gain in women who had less sleep.
"We don't have an answer from this study about why reduced sleep causes weight gain, but there are some possibilities that deserve further study," Patel says. "Sleeping less may affect changes in a person's basal metabolic rate (the number of calories you burn when you rest). Another contributor to weight regulation that has recently been discovered is called non-exercise associated thermogenesis (involuntary activity, such as fidgeting.) It may be that if you sleep less, you move around less, too, and therefore burn up fewer calories."

Another important factor to consider, as discussed in my book, The Thyroid Diet: Manage Your Metabolism for Lasting Weight Loss, , is the impact of sleep on cortisol levels. Insufficient sleep can cause the release of additional cortisol -- the stress hormone -- and can stimulate hunger.

According to the National Sleep Foundation, the average woman gets only six and a half hours of sleep per night. Chronic sleep deprivation can have a variety of effects on the metabolism and overall health.

Inadequate sleep:

• interferes with the body's ability to metabolize carbohydrates and causes high blood levels of glucose, which leads to higher insulin levels and greater body-fat storage.
• drives down leptin levels, which causes the body to crave carbohydrates.
• reduces levels of growth hormone--a protein that helps regulate the body's proportions of fat and muscle.
• can lead to insulin resistance and contribute to increased risk of diabetes
• can increase blood pressure
• can increase the risk of heart disease

Even in young, healthy people, a sleep deficit of three to four hours a night over the course of a week has a triple-whammy effect on the body.

> Find out more about the effects of sleep deficit on metabolism and health