Weight may depend as much on when you eat as what, research suggests.
The body clock's effect on metabolism could be an overlooked factor driving obesity, say scientists.
New evidence from studies of mice suggests that 24-hour snacking, especially at night, can pile on the pounds.
Restricting eating to sensible meal times, on the other hand, may help fight the flab - even with big helpings.
Researchers compared mice fed the same amount of high-fat food round the clock or over a period of eight hours.
The mice given a restricted time in which to eat were protected against obesity, and also suffered less liver damage and inflammation.
Lead scientist Dr Satchidananda Panda, from the Salk Institute for Biological Studies in the US, pointed out that every organ has a "clock".
Livers, intestines and muscles worked at peak efficiency at certain times and were more or less sleeping at others.
These metabolic cycles were critical for a wide range of biological processes, from cholesterol breakdown to glucose production.
"When we eat randomly, those genes aren't on completely or off completely," said Dr Panda.
He added there was evidence that eating patterns had changed, with people having greater access to food and reasons to stay up late, for instance to watch TV. When people were awake, they tended to snack.
The timing of food consumption should be given more consideration by obesity experts, said Dr Panda.
"The focus has been on what people eat," he said. "We don't collect data on when people eat."
The research is published in the journal Cell Metabolism.