Why you should care
Because there’s a reason Monday mornings are so hard — if you’re like almost 70 percent of people, then you’re suffering from social jet lag. It’s time to rethink our collective schedule.
Mornings are hard for many of us. Very few people wake up for work without an alarm clock, and we joke (but aren’t really joking) about needing that first cup of coffee in order to function. But it doesn’t have to be this way. We set the ”social clock,” and now, thanks to the data from the Munich Chronotype Questionnaire, we have reason to believe that starting our days a little later isn’t just a matter of personal comfort — but public health.
Number of clocks that govern your life
No, your smartphone is not the only clock that shapes your life; we are in fact bound by three different clocks. The solar clock transitions us from day to night and back again. Our biological clocks, or circadian rhythms, govern any number of biological functions — most obviously, sleep — and sync each day with the solar clock as our brains process light through our eyes. And finally, there is the social clock, that which divides our days into measured hours, and according to which our work, school, social and other calendars operate.
Once upon a time, humans’ social clocks more or less mapped onto the solar and biological clocks, and as the above number suggests, it was not terribly important for the “official time” that dictated the social clock in, say, New York, to exactly match the official time in Philadelphia. And then, to grossly oversimplify a much more complicated story, the industrial revolution happened, and electricity made it possible for us to brightly light our homes into the night, and trains started to run according to published timetables. Lo and behold, our social clock began to break with the solar clock more dramatically than ever before, and now very few of us actually rise and set with the sun.
Number of official times in North America in 1800
But what does this shift mean for that third clock, our circadian rhythms? It turns out that the degree to which people’s circadian rhythms correspond to the solar clock varies, and that while this variation is influenced by a number of environmental and social factors, some of that variation is genetic. That is, the difference between proverbial “larks” and ”owls” may have to do with different ”chronotypes,” or sleep time preferences. Whether or not you have an earlier or later chronotype — whether you’re a lark or an owl — impacts how well your body adjusts to the social clock.
The implications of different chronotypes have been explored most thoroughly by Till Roenneberg and his colleagues at the University of Munich, who through their Munich Chronotype Questionnaire (MCTQ) have examined sleep data from some 150,000 respondents around the world since they began collecting data in 2002. What the MCTQ has revealed is that later chronotypes do not map very well onto normal work schedules, and while chronotypes shift with age, most of us have chronotypes that are better suited to starting the day later than our social schedules typically demand on a workday.
12:30 a.m. – 8:30 a.m.
The median chronotype’s sleeping hours
Percentage of people whose circadian rhythms are not aligned to their workday
Later chronotypes – the owls or even just the semi-owlish – typically rise with alarm clocks that go off before they are fully rested, but they still don’t go to bed (in part because they cannot fall asleep) until later than the morning larks. So they accumulate “sleep debt” over the course of the workweek, which they then pay off with lots of extra sleep on the weekends. Paying off this sleep debt, though, results in dramatic shifts in a person’s circadian rhythms, and sleeping in means that your brain doesn’t cue your body to sync with that solar clock until several hours later than it would on a workday. The result? Social jet lag for – according to a 2012 report on MCTQ findings – a lot of people:
Percentage of people demonstrating at least one hour of social jet lag
Percentage of people demonstrating two or more hours of social jet lag
This is a problem because social jet lag results in some of the same symptoms as actual jet lag in the short term, including problems with a range of cognitive functions (most notably concentration and memory) and biological functions like digestion. In the long term, though, the ongoing mismatch between circadian rhythms and social clocks that social jet lag describes can lead to even more serious issues: higher risk of heart disease, obesity and increased cigarette and alcohol use. And, the MCTQ data reveals, the mismatch is getting worse.
Intriguingly these 40 minutes of sleep seem to be getting knocked off at the front end of our sleep — that is, respondents to the MCTQ survey were going to sleep 40 minutes later and getting up at the same time. Roenneberg and his team posit that this has to do with the corresponding 40-minute decrease in sunlight exposure that has taken place over the same period of time. The overwhelmingly strongest biological wake-up cue, the trigger for our brains and bodies to function properly, remains sunlight, and even the most well-lit interiors fall far short in matching exterior light intensity.
Number of minutes by which average workday sleep time has decreased in the past decade
It would be difficult to tabulate the social cost of all that sleep debt — the tasks that are performed more slowly, the answers that are forgotten, the information that teenagers and college students struggle to learn during morning classes … not to mention the general misery that many of us share on Monday mornings. But especially as the public health risks of social jet lag become more firmly established and obesity becomes a global challenge, we would do well to consider how to bring our social clocks in line with our biological ones, even if there’s no going back to the rule of the solar clock. Coming in to work or class at 10:00 is starting to sound downright responsible.