I used to think that if I slept less, I would burn more calories and therefor be more likely to lose weight. I read some articles today on the relationship between "sleep" and "weight loss" and completely re-educated myself.
Apparently, the optimum amount of sleep for losing weight is 7-9 hours. Any less than seven and your dopamine (the sense of "well-being" hormone) levels fall. One of the substances that supplies dopamine to your system is - you guessed it - sugar. So you are much more likely to crave sweets - and fats (another substance that scores high in the pleasure centres) - when you're tired. (And you thought it was only because you were looking for something to give you more energy!)
There's actually a double whammy, because there's another hormone level that is affected when you're tired. The levels of grehlin go up when you're lacking sleep. Grehlin is the stuff that tells your body you're really hungry. But at the very same time, your leptin levels are falling. Leptin is the hormone that helps your body acheive a sense of "full". So at the same time you're feeling hungrier with a strong sense of "more-ish", and craving sugar and fat, your lack of leptin is making you feel like a bottomless pit that can't ever be filled.
Good information to have. And a good reason to go to bed on time. (You'll live longer too!)