New findings suggest that staying up late and sleeping in on the weekend could negatively affect our metabolic health.
A team from the Medical Research Council in Oxford, U.K., looked at the impact of “social jet lag,” a term used to describe the difference in people’s sleep patterns between work days and free days. An estimated 87 percent of the population suffers from social jet lag to some extent.
The team used figures from 815 non-shift-working participants on the long-term Dunedin Multidisciplinary Health and Development Study. Participants were all born between April 1972 and March 1973 in New Zealand, and followed from ages three to 38, when 95 percent of the 1,007 study members were still alive.
Participants’ height, weight, and waist circumference were measured, along with C-reactive protein, a biomarker for inflammation, and glycated hemoglobin in the blood, a marker for diabetes. Measurements were combined with results of a questionnaire on sleep duration and the individual’s preference in sleep timing, known as “chronotype.”
Social jet lag was measured by subtracting each person’s midpoint of sleep on work days from their midpoint of sleep on free days (assuming five work days and two free days a week as standard). So, for example, if someone slept from 12:00 a.m. to 8:00 a.m. on workdays, the midpoint was 4:00 a.m. If they then slept from 1:00 a.m. to 11:00 a.m. on free days, the midpoint was 6:00 a.m., giving a social jet lag of two hours.
Findings showed that those with a greater difference in sleep patterns on free days and work days had a significantly higher risk of obesity and obesity-related disease, including metabolic disorder (the medical term for a combination of diabetes, high blood pressure, and obesity) and inflammation. Just a two-hour difference in sleep patterns on the weekend was linked to risk of an elevated body mass index and risk of inflammation and diabetes.
Dr. Michael Parsons, lead author, explains that while travel jet lag can cause temporary problems with metabolism, social jet lag “can occur chronically throughout an individual’s working life so is more likely to induce more serious, chronic consequences for metabolism.”
He adds, “Social jet lag is an under-researched but potentially key contributor to why ‘living against our internal clock’ has an impact on our health. Our research confirms findings from a previous study that connected people with more severe social jet lag to increases in self-reported body mass index, but this is the first study to suggest this difference in sleeping times can also increase the risk for obesity-related disease.”
Full details appear in the International Journal of Obesity. The team points out, “Obesity is one of the leading causes of preventable death worldwide.” They are unsure why social jet lag may cause the raised risk, but suggest it may be that it disrupts healthy habits such as diet and exercise in a way that may compromise health.
Co-author Dr. Terrie Moffitt believes “[T]hese findings help us start to actually understand the physiology of social jet lag and how it impacts upon obesity and obesity-related disease. Further research that determines this association could help inform obesity prevention by influencing policies and practices that contribute to social jet lag, such as work schedules and daylight savings.”
The research was partly funded by the U.K.’s Medical Research Council.
Professor David Lomas of the council commented, “This study adds further evidence to previous research that living against our body clock, even if just on a small scale, may be part of the problem behind rising obesity and related disease. It could allow for exploration of how changes to our diary as well as diet, could help to reduce this upwards trend and improve public health.”
The term “social jet lag” was first coined in 2006 by Dr. Till Roenneberg, a professor at the Institute of Medical Psychology at the University of Munich, who used it to describe the difference between midsleep on free days and midsleep on work days.
Although the phenomenon has been linked to a range of health risks, the studies have mainly been correlational and therefore, it is possible that the reverse may be true — obesity and associated health risks and conditions may cause this sleeping pattern.
Parsons, M. et al. Social Jetlag, Obesity and Metabolic Disorder: Investigation in a Cohort Study. The International Journal of Obesity, 20 January 2015 doi:10.1038/ijo.2014.201