Being slimmer could be as simple as waking up early for some morning sunshine, US researchers suggested on Wednesday.

A small study of 54 volunteers showed that the leanest ones did not necessarily eat better or exercise more than the rest. They simply were exposed to more bright daylight earlier in the day.

The findings by Northwestern University researchers are published in the journal PLOSONE.

"The earlier this light exposure occurred during the day, the lower individuals' body mass index," said co-lead author Kathryn Reid, research associate professor of neurology at Northwestern University Feinberg School of Medicine.

The study found morning light accounted for about 20 percent of a person's body mass index (BMI).

"The message is that you should get more bright light between 8 am and noon," she added.