Night owls may be more prone to heart disease and diabetes than early birds because their bodies are less able to burn fat for energy, US researchers say.

People who rise early rely more on fat as an energy source, and are often more active in the day, than those who stay up later, meaning fat may build up more easily in night owls, the scientists found.

The findings may help explain why night owls are at greater risk of type 2 diabetes and cardiovascular disease, and may help doctors to identify patients early on who are more likely to develop the conditions.

“This could help medical professionals consider another behavioural factor contributing to disease risk,” said Prof Steven Malin, a senior author on the study and expert in metabolism at Rutgers University in New Jersey.

The researchers divided 51 obese middle-aged adults into early birds and night owls, depending on their answers to a questionnaire on sleeping and activity habits. They monitored the volunteers’ activity patterns for a week and tested their bodies’ fuel preferences at rest and while performing moderate or high-intensity exercise on a treadmill.

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Writing in Experimental Physiology, the team describe how early birds were more sensitive to blood levels of the hormone insulin and burned more fat than night owls while at rest and during exercise. The night owls were less sensitive to insulin and their bodies favoured carbohydrates over fat as an energy source.

Malin said it was unclear why differences in metabolism were seen in night owls and early birds, but one possibility, he believes, is a mismatch between the time people go to bed and wake the next morning and the circadian rhythms that govern their body clocks.

“Night owls are reported to have a higher risk of obesity, type 2 diabetes and cardiovascular disease when compared with early birds,” he said. “A potential explanation is they become misaligned with their circadian rhythm for various reasons, but most notably among adults would be work.”

If a person is a night owl, Malin added, they may prefer to go to bed late but still have to get up early to go to work or to look after children, and this may force them to be out of alignment with their body clocks when they would rather be sleeping.

The findings could affect discussions around the health risks of night-shift work and even changing the clocks to suit daylight hours. “If we promote a timing pattern that is out of sync with nature, it could exacerbate health risks,” Malin said. “Whether dietary patterns or activity can help attenuate these is an area we hope becomes clear in time.”