From a paper in the latest issue of Current Biology, I learned that the habit of many people to switch their sleeping hours between a later weekend time and an earlier work / school time is now referred to as social jetlag in the chronobiological literature. The latest research links this seemingly innocuous behaviour pattern to obesity.
The more extreme variant, the lifestyle of shift workers, has been studied in some detail before, and it is well known that it has serious health implications, as our metabolism works according to our inbuilt circadian rhythm, not by our alarm clock and work schedule. I believe that the investigations have led to some improvements and people who want to soften the blow can treat their jet lag with the clock hormone melatonin, but I’m not sure how widely the problem is appreciated among people working variable shifts and among their employers.
The more widespread version of just having your sleep two or three hours later on weekends than on weekdays (equivalent to a return trip across several timezones each weekend) hasn’t been appreciated as a health risk so far, but the authors of the new study describe it as “chronic social jetlag” and show that there is a statistically significant link with obesity, at least for some who are sensitive to weight gain.
Intriguingly, those in the normal range of body mass index (BMI), there was no significant correlation between the number of hours of social jetlag (up to 5 hours – that must be stressful!). For those in the overweight to obese range however, the BMI appears to correlate positively with social jetlag above 1h.
There are some very obvious measures that could reduce this problem instantaneously, for instance, adapting school hours to the (typically late) chronotype of teenagers, but will anybody pay attention? I doubt it. Personally I’m in the clear – my weekend timeshift is typically less than half an hour, and I’m not overweight anyway.
Till Roennebergsend, Karla V. Allebrandt, Martha Merrow, Céline Vetter: Social jetlag and obesity
Current Biology, Volume 22, Issue 10, 939-943, 10 May 2012 10.1016/j.cub.2012.03.038