Why does lack of sleep lead to weight gain?

According to a new study, lack of sleep is linked to weight gain; the researchers suggest that changes in the levels of two hormones linked to appetite are to blame. Is that the only possible explanation?

An alternative explanation is that people who sleep less are awake more. Since people eat when they are awake, people who are awake more have more opportunity to eat, so it's possible that in fact they do eat more, and the added food intake exceeds the energy needed to spend a few more hours awake. The hormonal changes may be simply adaptations to trigger a modified eating schedule and thus cope with more up-time.