Researchers found that after taking lifestyle factors into account, people's habit of going to bed later and getting up later was associated with a 19% increased risk of diabetes. A new study has important information for those who consider themselves night owls. Researchers from Brigham and Women's Hospital, a founding member of the Massachusetts General Brigham Health System, found that people who go to bed later and get up later are at greater risk of developing diabetes than those who go to bed early and get up early. Their findings were published Sept. 12 in the Annals of Internal Medicine.
"Circadian rhythm, or circadian rhythm preference, refers to a person's preferred sleep and wake times, which is determined in part by genes and therefore may be difficult to change," said corresponding author Huang Tianyi, MS, PhD, associate research fellow in epidemiology at the Brigham Channing Network's Department of Medicine. "People who consider themselves 'night owls' may need to pay more attention to their lifestyle because their evening chronotype may increase their risk of type 2 diabetes."
Researchers have previously found that people who sleep more irregularly have a higher risk of diabetes and cardiovascular disease, while people with an evening chronotype are more likely to have irregular sleep patterns. In this study, they wanted to understand the relationship between chronotype and diabetes risk and examine the role of lifestyle factors.
The team analyzed data from 63,676 female nurses in the Nurses' Health Study II collected between 2009 and 2017, including self-reported chronotype (the extent to which participants considered themselves to be late or early bedtimes), diet quality, weight and body mass index, sleep duration, smoking behaviour, alcohol consumption, physical activity and family history of diabetes. The research team determined participants' diabetes status based on their self-reports and medical records.
The Nurses' Health Study II, a joint study of the Brigham Channing Network Medicine and the Harvard T.H. Chan School of Public Health, is one of the largest surveys of major chronic disease risk factors in women. One of the strengths of the study is that study participants were followed regularly and health and lifestyle factors were repeatedly assessed.
About 11% of the participants said they had a "definite evening" time type, and about 35% said they had a "definite morning" time type. About half of the remaining people are called "intermediate," meaning they are either neither a morning nor an evening type, or just slightly more of one than the other.
Before taking lifestyle factors into account, "evening type" was associated with a 72% increased risk of diabetes. After accounting for lifestyle factors, "evening type" people had a 19% increased risk of developing diabetes. In the study, only 6% of people with the healthiest lifestyles had a twilight syndrome. Among those with the least healthy lifestyles, 25% have late-onset disease.
The study found that people with a late-morning lifestyle were more likely to drink heavily, have a lower-quality diet, get fewer hours of sleep each night, be current smokers, and be in unhealthy ranges for body weight, body mass index, and physical activity rates.
"When we controlled for unhealthy lifestyle behaviors, the strong relationship between chronotype and diabetes risk was attenuated but remained," said first author Sina Kianersi, DVM, a postdoctoral fellow in the Division of Medicine at the Brigham Channing Network.
They also found that the association between evening chronotype and diabetes risk was only among those nurses who worked the day shift, but not those who worked the night shift.
"We found an increased risk of type 2 diabetes when chronotype does not match work hours," Huang said. "This is another very interesting finding that suggests a more personalized work schedule may be beneficial."
The Nurses' Health Study consisted primarily of white female nurses—future investigation is needed to determine whether the patterns found here are consistent across different populations. The findings suggest correlation but not causation -- there may be other factors that contribute to a person's chronotype, tendency toward unhealthy habits and diabetes risk.
Next, the researchers plan to investigate the genetic determinants of chronotype and its association with cardiovascular disease and diabetes in a larger, more diverse population.
"If we can determine the cause-and-effect relationship between circadian rhythms and diabetes or other diseases, doctors can better tailor prevention strategies to their patients," Kianersi said.