Changes in the daily pattern of a hormone which controls hunger could explain why some people become obese, US scientists have claimed.
A University of California study looked at levels of the hormone ghrelin in five lean and five obese men.
Researchers found the ghrelin levels of lean men peaked during the night but in obese men it rose most during the day.
This meant obese men were more likely to eat, they told the Proceedings for the National Academy of Sciences.
Mealtime peaks
Ghrelin is made by cells in the stomach and is the only known hormone that stimulates hunger. Its levels go up before meals and fall again after eating.
Dr Julio Licinio's team looked at the ghrelin levels of five lean and five obese young men.
They took blood from the men every seven minutes over 24 hours.
The lean men had a large increase in ghrelin levels between midnight and 6am.
This was larger than the spikes of ghrelin seen before meals.
In comparison, the obese men had no night-time peak and had overall lower levels of ghrelin.
Dr Licinio said at first glance their findings appeared to be contradictory.
"You'd expect the blood levels of the heavier men to contain more hunger hormone. Something must be overriding obese persons' ghrelin," he said.
"The most powerful ghrelin surge was missing in the obese men, suggesting that their regulatory system has gone awry or is no longer able to listen to its own cues," he said.
For the obese men, the predominant rises in ghrelin were during the day when they were awake and able to eat.
Dr Licinio's team also measured the men's blood levels of two hormones produced by fat cells - leptin and adiponectin. Leptin tells the body when it is full and adiponectin helps regulate energy.
The overweight men had higher levels of leptin and lower levels of adiponectin than the lean men but, unlike ghrelin, the hormones followed the same fluctuations in both groups.
Over-eating
Dr Licinio said the findings defied the stereotype of overweight people waking up in the middle of the night to raid the refrigerator.
"The men in our study slept through the night and both groups ate meals designed to maintain their current weight," he said.
"For someone to gain weight and become obese they must eat a lot. So this could help explain why obese people eat more during the day. Ghrelin is stimulating appetite during the day," he said.
He added that it might be possible to treat obesity by targeting ghrelin.
"We must try to solve this mystery and explore new drugs to make them more sensitive to their bodies' internal cues.
"Our next step is to see what's happening during weight gain and weight loss," he said.
National Obesity Forum chairman Dr David Haslam said: "It's very interesting and there is a certain degree of potential there but in practical terms we need to see a lot more research on a lot greater population of people.
"If and when that happens we will welcome it with open arms as a potential weapon against obesity," he said.