Researchers at the University of Bristol have found that hormonal changes caused by lack of sleep could lead to increased appetite.
The population-based study examined more than 1,000 volunteers.
Report author Dr Shahrad Taheri said: "Individuals who spent less than eight hours sleeping were shown to have a greater likelihood of being heavier."
It found that people who habitually slept for five hours had 15% more ghrelin, a hormone which increases feelings of hunger, than those who slept for eight hours.
Those who slept for less time were also found to have 15% less leptin, a hormone which suppresses appetite.
"These differences are likely to increase appetite and, in societies where food is readily available, this may contribute to obesity," Dr Taheri said.
"It is important for people to realise there is more to obesity than just stuffing your face."
This study found people who slept for only four hours a night for twonights had an 18% reduction in leptin, anda 28% increase in ghrelin.
The young men in the study also tended to eat more sweet and starchy foods when sleep was cut short.
A third recent study, by Columbia University, also found found people who slept four hours or less per night were 73% more likely to be obese.