Need another reason to loathe your alarm clock? Waking each workday to the jolt of an alarm, then sleeping in on weekends might increase your risk of diabetes and heart disease, a U.S. study suggests.
Researchers looked at what's known as social jet lag, a term they used to describe following one sleep schedule on workdays and another on days off. Much as a cross continental flight can disrupt circadian rhythms by hurling passengers abruptly across several time zones, social jet lag upsets the body's biological clock.
"Social jet lag is a habitual form of circadian misalignment, when individuals have to essentially sleep and wake at times that are out of synch from their internal, biological clock and shift back and forth in their sleep schedules due to social obligations," lead study author Patricia Wong of the University of Pittsburgh said by email.
When Wong and her colleagues looked at a group of about 450 middle-aged adults, they found the people with the biggest shift in sleep routines between workdays and days off were also most likely to have health problems that are risk factors for diabetes and heart disease.
These symptoms included extra girth around the midsection and higher levels of sugar and fats in the blood, as well as low levels of high density lipoprotein (HDL) cholesterol - the good kind of cholesterol that helps prevent damage to blood vessels.
The results don't prove social jet lag causes diabetes or heart disease, but it does suggest the connection merits a closer look, Wong said.
To explore the connection between social jet lag and risk factors for diabetes and cardiovascular disease, Wong's team studied a group of adults aged 30 to 54 years who were working at least 25 hours a week outside the home.
The researchers excluded people with a whole host of medical conditions that might contribute to risk factors for diabetes and cardiovascular disease, such as people who had been treated for renal failure, lung diseases, hypertension, mental illness, diabetes, obesity or high cholesterol. They also excluded shift workers.
Researchers asked participants to wear devices to track their activity and sleep around the clock for seven days. The devices registered sleep duration as well as the midpoint of total sleeping time for each day, including workdays and time off.
Most participants - 85 percent - experienced the midpoint of their sleeping time later on days off than on workdays. For most, this was a result of going to sleep later on days off, sleeping longer or doing both. About 15 percent of the people in the study showed the opposite pattern, with the midpoint of sleeping time occurring earlier on days off.
People who were typically awake later at night, known as an evening chronotype, were more likely to have social jet lag and also more apt to be younger and poorer. These people also tended to have lower HDL cholesterol, as reported in Journal of Clinical Endocrinology and Metabolism.
One shortcoming of the study is that researchers lacked data to see if people with more pronounced social jet lag or evening chronotypes had different circadian rhythms than other people, the authors concede. In addition, while researchers tracked food intake, they didn't monitor what people ate based on the time of day, which can influence many cardiometabolic risk factors, the authors note.
Even so, the findings add to a growing body of evidence suggesting that social jet lag, with a routine shifting in and out of different routines based on work schedules, may be even more damaging than ordinary jet lag from travel that allows people to adjust to a new routine and then stick to it, said Dr. Till Roenneberg, a psychology researcher at the University of Munich in Germany, who wasn't involved in the study.
"We had already known from various epidemiological and experimental studies that metabolism - and especially glucose/insulin metabolism - is challenged by living against one's clock," Roenneberg said by email. "The links between body mass index, metabolic syndromes, and cardiovascular pathologies are well established, so this new report was a welcome support of earlier findings."