Most of us are familiar with the short-term costs of sleep deprivation: reduced alertness, daytime sleepiness, irritability, and trouble concentrating. Long-term sleep loss can have more serious health effects, including depression, hypertension, health problems, and stroke. Now research suggests that insufficient sleep also contributes to obesity. Although the mechanism isn’t entirely clear, the evidence implicates hormones that control appetite. If further studies confirm these findings, getting enough sleep could emerge as a valuable addition to exercise and diet in achieving and maintaining a healthy weight.

Less sleep, higher body mass index

Several studies have examined the relationship between sleep and body weight. In one, researchers at Eastern Virginia Medical Center recruited 924 women and men, ages 18–91, from local medical practices and interviewed them about sleep habits, health problems, and sleep disorders. Weight and height measurements were taken and subjects were classified by body mass index, or BMI.

The higher a person’s BMI, the less sleep she or he got. The relationship held even after excluding subjects with breathing problems, which are often caused by excess weight. Overweight and obese subjects slept, on average, 1.86 fewer hours per week — almost 25 minutes less per night — than normal-weight subjects. This sleep deficit correlated with a significant difference in their BMIs.

The hormone connection

How might insufficient sleep affect body weight? Research conducted in sleep labs shows that sleep deprivation can alter metabolic functions such as the processing and storage of carbohydrates. It also influences the activity of several hormones, including leptin, which suppresses appetite.

Scientists at Stanford University and the University of Wisconsin hypothesized that ghrelin (GRAY-lin), an appetite-stimulating hormone, might be involved as well. To investigate, they examined data from a long-term study of sleep habits and disorders in 1,000 volunteers. The data, collected over 15 years, included surveys, overnight sleep studies, blood samples, and sleep diaries.

In people who slept less than eight hours per day, BMI rose in direct proportion to decreases in sleep time. Further, people who slept less had elevated ghrelin and reduced leptin, a combination that can result in hunger.

What now?

According to data from the National Sleep Foundation, most adults in the United States sleep about seven hours per night — one hour less than health experts recommend. Occasional sleep loss isn’t a problem, but a sustained sleep debt can negatively impact health. If getting enough sleep is a problem for you, discuss it with your clinician.

There’s no proof that insufficient sleep contributes to obesity. Nor is it likely that it’s as important as diet, exercise, and genetics. But the relationship merits further investigation. If there is a connection, adequate sleep might be recognized as a factor in successful weight loss. In any case, many of us know from experience that lack of sleep tends to undermine our good intentions to eat well and exercise.

For more information on sleep, order our special health report, Improving Sleep: A guide to a good night’s rest. www.health.harvard.edu/IS.

Q: In the past year, I have been under a lot of stress, and I have been breaking out in hives and skin rashes. Is there a link between stress and hives?

A: Emotional stress probably can cause hives and other skin reactions.

Hives, also called weals, are raised, itchy swellings of the skin. Most weals last no more than 24 hours, but attacks can last longer because new ones erupt as the older ones fade. For many years doctors have observed that hives erupt under stress, but clear scientific evidence for the connection has been hard to find.

Hives develop when a type of immune cell called a mast cell causes the release of the chemical histamine. It is still a mystery how psychological stress triggers this immune response. One possibility is that nerve cells produce a substance that stimulates histamine activity. No one knows how much stress is necessary, or why one person is more vulnerable than another.

Given all the unanswered questions, a practical approach is best. If you address anxiety and stress with anti-anxiety medication, psychotherapy, stress management, and relaxation techniques, skin reactions may become less frequent. But hives can have many other causes, so a general medical evaluation is also important.

Michael Craig Miller, M.D., is Editor in Chief of the Harvard Mental Health Letter (www.health.harvard.edu/mental)