(REUTERS) - Timing meals relative to your own body clock, rather than to the time of day, may affect how lean you are, researchers suggest.
Studies have shown that eating later in the day ups your risk of weight gain.
However, the impact of a person's body (biological) clock - independent of the time of day - has not been tested until now, according to Dr Andrew McHill of Brigham and Women's Hospital in Boston and colleagues.
"Our findings could be considered a reason not to eat right before going to sleep, but they're also a reason not to eat later in the evening, even if you are planning to go to bed at a later time," Dr McHill told Reuters Health.
The team recruited 110 college students aged 18 to 22 (about 60 per cent male) for a 30-day study of sleep times and food intake.
The students completed questionnaires about their sleep habits at the outset of the study, as well as daily electronic sleep-wake and exercise diaries. They also wore motion monitors throughout the study to help track sleep-wake timing.
For one week during the study, participants used a mobile phone app to time-stamp, document and record their food intake during their regular routines.
They were also evaluated for one night at the hospital to see what time their level of the hormone melatonin began to rise - which marks the beginning of a person's biological night - and to assess their body composition (ie muscle mass and fat).
Melatonin onset timing was similar for both lean participants and those with a higher percentage of body fat, according to the American Journal of Clinical Nutrition study, online Sept 6.
However, those with a higher percentage of body fat - 8.7 per cent higher in women and 10.1 per cent higher in men - ate most of their calories about an hour closer to the time of melatonin onset than did lean participants.
There was no relationship between body composition and when (clock hour) they ate, how many calories they consumed, what kind of food they had, their exercise or activity level or sleep duration.
"While it's not possible to know the timing of your melatonin onset without having it measured very precisely in dim lighting, we tend to think that melatonin levels rise about two hours prior to habitual sleep onset," Dr McHill explained.
What about waking up and eating a snack in the middle of the night?
"This would also be a time when melatonin is high and your body clock is promoting sleep and fasting," he said, "so we would consider that a time that food consumption could lead to higher body fat if done repeatedly over a long period of time."
Dr McHill cautioned that the findings do not show cause and effect. To do that, he said, "randomised controlled trials that include altering the timing of meals of the exact same food content in relation to melatonin timing (eg providing meals within four hours of melatonin onset or restricting calories to when melatonin concentrations are low) are needed."
It is also important to study groups other than college students and the team has already begun to track meal timing in older and ill populations.
Dr Eric Feigl-Ding, a nutritional epidemiologist at Harvard Chan School of Public Health in Boston who was not involved in the study, told Reuters Health he agrees that "the takeaway is that eating earlier before bed may be better" - perhaps as much as four to five hours earlier.
However, "actual experiments to show direct long-term weight loss and health benefits from consistently eating earlier before bed are needed", he added by e-mail.
"Be vigilant of your food intake as time to sleep approaches," Dr Jocelyn Cheng, a neurologist at NYU Langone Health in New York City urged in an e-mail to Reuters Health.
"If you notice yourself eating more during this period compared to earlier in the day, consider redistributing your meals, snacks included," said Dr Cheng, who was not involved in the study.