Some people are early birds. They have a morning chronotype or circadian preference: They tend to naturally go to bed early and wake up early in the morning, and have more energy early in the day.

Then there are those with an evening chronotype. They tend to go to sleep later, are more likely to sleep late, and have more energy later in the day. These are the night owls.

Night owls had a 72 percent higher risk of developing diabetes during the eight-year follow up period.

Chronotype is partly determined by genetics, and about eight percent of the population have an evening chronotype. Night owls are more likely than early birds to have poor metabolic and blood sugar control, metabolic disorders and a greater risk of type 2 diabetes. The reasons for the link between evening chronotype and type 2 diabetes risk are not well understood, however.

A recent study looked at the health effects of an evening chronotype in a group of nurses who often worked the night shift over eight years.

The prospective study used data from more than 63,000 women in the Nurses Health Study II. The women, 45 to 62 years old and with no history of cancer, cardiovascular disease or diabetes at baseline, were followed over an eight-year period. Participants responded to a questionnaire to determine if they were a morning chronotype or an evening chronotype.

Researchers from Brigham and Women's Hospital and Harvard T.H. Chan School of Public Health and Harvard Medical School found that night owls had a 72 percent higher risk of developing diabetes during the eight-year follow-up period.

They also noted that night owls were 54 percent more likely than early birds to report unhealthy lifestyle habits. They were more likely to smoke, abuse alcohol, and get too little sleep and physical activity. Night owls also tended to have a poor diet and a high body mass index.

Interestingly, taking these habits into account reduced the risk of diabetes among those women with a late night chronotype, but it did not completely eliminate it. More research is needed to determine whether chronotype is a causal risk factor for diabetes or a reflection of a cluster of lifestyle factors.

It is possible that reassigning night owl workers to night shifts could put them more in sync with themselves and improve their sleep habits and possibly reduce their diabetes risk, according to an editorial related to the study, which suggested, “[L]ifestyle interventions and personalized shift scheduling may have health benefits beyond diabetes prevention.”

Going forward, more information, including tools to evaluate a person's chronotype throughout their life, is needed. As the authors of the editorial point out: “Chronotype can change as people age, so a single assessment may not accurately capture the cumulative effect of long-term circadian misalignment.”

It is not yet clear whether being a night owl is a cause of diabetes or the reflection of a cluster of lifestyle factors.

Almost all of the women in the study were middle-aged and white. Other studies are needed to see if these results apply to men, as well as people from other racial, ethnic and socioeconomic groups. Age may also play a role, so studies with younger or older populations are necessary, too. The findings may not apply to younger or older people because their diet and exercise habits and their body weight likely differ from those of the current study population.

The study and the related editorial are published in the Annals of Internal Medicine.