Jillian Michaels may disagree, but researchers from the University of Washington found that getting a lot of sleep may not lead to weight gain.  

According to the study, getting more than nine hours of sleep each night may actually turn off some of the genetic activity linked to body weight and weight gain.  This is because lifestyle factors, such as diet and exercise, have a larger impact on body weight than sleep duration.  In other words: you generally cannot overeat when you are sleeping and overeating is more likely to cause weight gain than hitting the snooze button. 

Researchers stress, however, that most adults need seven to nine hours of sleep each night for optimal health.  Too little or too much sleep is going to have a negative impact beyond simple weight gain.