For the millions of Americans who have resolved to lose weight this year, success may depend on how much they sleep.

Studies published in the Journal of the American Medical Association (JAMA) and the Lancet suggest that sleep loss may increase hunger and affect the body’s metabolism, which may make it more difficult to maintain or lose weight.

Sleep loss has been shown to affect the secretion of cortisol, a hormone that regulates the appetite; as a result, individuals who lose sleep may continue to feel hungry despite adequate food intake, JAMA said. In addition, sleep loss may interfere with the body’s ability to metabolize carbohydrates and cause high blood levels of glucose, a basic sugar; excess glucose promotes the overproduction of insulin, which can promote the storage of body fat and also can lead to insulin resistance, a critical feature of adult-onset diabetes, according to the JAMA report.

"Sleep loss is associated with striking alterations in hormone levels that regulate the appetite and may be a contributing factor to obesity," said Dr. Michael Thorpy, director of the Sleep-Wake Disorders Center at New York City’s Montefiore Medical Center. "Any American making a resolution to lose weight in the New Year should probably consider a parallel commitment for getting more sleep."

According to national research statistics, weight loss in the number-one New Year’s resolution in America, with approximately 40 percent of the population promising to diet. A nationwide survey found that more than 75 percent of women between the ages of 25 and 54 make diet resolutions almost every year. However, nearly 90 percent of the respondents reported either occasional or no success, with almost half losing little weight or actually gaining weight, the survey found.

"Sleep loss disrupts a complex and interwoven series of metabolic and hormonal processes and may be a contributing factor to obesity," said Dr. John Winkelman, medical director of the Sleep Health Center at Brigham and Women’s Hospital in Boston and assistant professor of psychiatry at Harvard Medical School. "What most people do not realize is that better sleep habits may beinstrumental to the success of any weight management plan."

The National Sleep Foundation recommends that people who vow to lose weight this year should adjust their sleep habits and their eating habits. The foundation says the following tips will help Americans keep their resolution on track:

* Don’t go to bed feeling hungry, but don’t eat a big meal right before bedtime.

* Exercise regularly, but do so at least three hours before bedtime.

* Avoid caffeine, nicotine and alcohol in the late afternoon and evening.

* If you have trouble sleeping at night, don’t nap during the day.

* Establish relaxing pre-sleep rituals, such as a warm bath or a few minutes of reading.

* Create a pleasant sleep environment. Make it as dark and quiet and possible.

* If you can’t sleep, don’t stay in bed fretting. After 30 minutes, go to another room and involve yourself in a relaxing activity until you feel sleepy.