1. The Hormonal Effect: Leptin is a hormone that reduces hunger. A recent study found that when participants slept only four hours a night they had lower levels of leptin than when they slept a full 10 hours. Further, the lack of sleep also caused the hormone that stimulates appetite to increase. (Double whammy.)
2. The Poor Food Choice Effect: People who are sleep deprived often consume higher amounts of high carbohydrate foods, (soft drinks, sweets etc), to get a needed energy boost. These high energy dense items can increase the daily caloric intake.
3. The Tired Leads to Lazy Effect: When exhausted, getting a good workout done is not super appealing. Exercise is often skipped, or if done, the intensity and/or duration may be reduced.
So to help your weight loss/maintenance, get off the web, get in bed, and get enough sleep.
BTW: How much is enough? The amount of sleep a person varies with many factors – including age. A general guideline for adults is 7-9 hours per day.
Sources: The National Institutes of Health, webmd.com, mayoclinic.com