How 2 Hours of Sunlight Could Fix Your Work Week

Till Roenneberg is a clock researcher. The clock he studies is the body clock, which means he’s an expert on sleep. He studies all kinds of factors that influence sleep, ranging from sunset times to genes. I was interested in his work because sufficient sleep is absolutely critical to personal productivity. To understand one, I certainly should put in some effort to understanding the other.

I just finished his book Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired (2012). I was especially interested in a few chapters that touched on the difference between city-dwellers (who are often knowledge workers) and rural people. Then I landed on this particularly striking passage:

“One of the questions in the Munich ChronoType Questionnaire [a long-running survey that is a major database for Roenneberg’s work] asks participants to indicate how much time they spend outside every day, without a roof above their head. It is remarkable how little time many people spend outside of buildings or some kind of vehicle. According to our survey, about half of the Central European population spends on average less than an hour outdoors on work days and less than three hours on weekends. When we correlated the time people spend outdoors with their chronotype we found a very systematic relationship that can well explain the differences in sleep-wake behavior between town and country folk.

“Based on these highly subjective self-assessments, spending two hours or more outdoors can advance an individual’s chronotype by about one hour [emphasis mine]. This means that if we were to cycle to work and back instead of taking the subway, we might become one hour less sleep-deprived every night during the workweek and would therefore have less sleep loss to compensate for on our free days. And we would also boost our learning capacity, our immune system, our mood, and our social skills [endnote with additional citations].”

In addition to reading the book, I picked up a few of Roenneberg’s earlier papers to get the real nerdy stuff. In a 2007 paper, he and his co-author write about the same effect:

“Chronotype progressively advances by more than an hour when people spend up to 2 hours outside per day; 42% of the population in our database fall into this category, stressing the fact that industrialization means living inside. Beyond 2 hours of natural light exposure, chronotype changes very little. This is important because the sleep debt accumulated by late types over the workweek … would be greatly reduced if those individuals could fall asleep an hour earlier by spending more time outside.”

When I read the passages above, I couldn’t believe I had never heard this effect discussed anywhere before.

Sleep is, of course, absolutely essential to productivity. We need sufficient sleep to focus, to make good decisions, to be creative, and so forth.

Internal Time does a thorough job of explaining why the exact amount of sleep that each person needs to function adequately is unique. He also explains that amount of sleep is only one factor. Ideal bedtime and wake times also vary for each person, and they are just as important. So allow me to unpack briefly his mention of “late chronotypes.” Late chronotypes are night owls. They are people who are predisposed to go to be later than others. Internal Time spends many chapters making the case why being a night owl is not wholly a product of social cues or preferences but is rather more like a personality trait that we are born with.

It is rare to hear a qualified researcher make suggestions for actions someone might take to improve his or her circumstances. I get very excited when they do. Roenneberg does not make the leap to say getting more sunlight will increase productivity, only that it may improve a night owl’s ability to fall asleep earlier on work days and therefore get more sleep. But because we know how important sleep is to productivity, I’ll make a baby-step leap and say that it certainly wouldn’t hurt.


Roenneberg, T. (2012). Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired. Harvard University Press. Cambridge, Mass.

Roenneberg, T., & Merrow, M. (2007). Entrainment of the Human Circadian Clock. Cold Spring Harbor Symposia on Quantitative Biology, Volume LXXII.

Image by a.has, CC.


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