"We found that insufficient sleep increased total daily energy expenditure by [approximately] 5 per cent; however, energy intake - especially at night after dinner - was in excess of energy needed to maintain energy balance," the researchers wrote in their paper. "Our findings suggest that increased food intake during insufficient sleep is a physiological adaptation to provide energy needed to sustain additional wakefulness; yet when food is easily accessible, intake surpasses that needed."
The study subjects overate despite the release of hormones signalling the body did not need extra energy from food, the researchers said.
The participants' food intake fell when they were allowed to sleep up to nine hours overnight and their weight dropped correspondingly.
Co-author, Associate Professor Kenneth P. Wright jnr, , said there were no insomniacs among his study subjects.
"We did not study patients with insomnia and our findings may not directly translate to the insomnia patient population."
Dr Siobhan Banks, senior research fellow, Centre for Sleep Research at the University of South Australia, said it was "a beautifully controlled study".
"They didn't just examine the hormonal consequences of sleep loss or what was eaten but also the energy expenditure."
The researchers tried to be realistic by making sure the study participants exercised a little and were exposed to normal amounts of daytime light.
People who regularly get insufficient sleep, such as shift workers or insomniacs, should try to limit food intake at night, said Dr Banks.
"There's a difference between fancying something for the psychological comfort and being hungry. That's something we always need to ask ourselves before a meal or snack. Am I really hungry?" she said.
"If you really are hungry, go for a small, protein-based snack."
Gaining weight might increase the risk for other comorbidities that might complicate insomnia, said Dr Banks.
"Being overweight might make you feel more anxious or depressed which could exacerbate insomnia and lead to a vicious cycle of weight gain and difficulty sleeping."
Professor Leon Lack, a sleep expert in the School of Psychology at Flinders University, said it was a well-designed study.
"It reinforces earlier studies but the careful control was able to isolate the weight gain to greater calorie intake than expenditure in the sleep restricted group.
"So, an imposed reduction in sleep by about three hours per night can lead to weight gain with all the possible increased health risks associated with that," he said.
"One question that now needs to be investigated is what would happen in a normal life situation if reduced sleep occurred as a result of increasing life demands ... ?
"This might result in an increased activity level and metabolic rate that balances increased food intake. This could be investigated in a longitudinal study where individuals encounter changing life demands [for example, high stress work alternating with holidays] in which both sleep length, activity levels, and probably metabolic rate all vary to mutually compensate each other."
Insomniacs, who are distressed by their poor sleep and often in stressful conditions don't seem to be overweight on average, despite their reduced amount of sleep, he said.
"I am a bit concerned that the results of this type of study may be interpreted too broadly and increase the anxiety of insomniacs about their shorter sleep, thus exacerbating their insomnia."
Sunanda Creagh is news editor of The Conversation and a former reporter at Reuters and the Sydney Morning Herald.
