Time Bound

Till Roenneberg takes a closer look at how our internal clock and sleeping pattern affect our lifestyle choices

Published 6 years ago on Mar 18, 2018 4 minutes Read

In circular time structures, simple, orderly sequences are difficult  to determine—the old chicken- and-egg problem. Is dawn before dusk or is dusk before dawn? Is midnight before noon or is noon before midnight? It seems easier when events lie closer to each other: dawn occurs before noon, dusk after noon. But even these apparent certainties are not necessarily correct. If the cooling-off at the end of the day influenced the weather on the next day, then the rain at lunchtime would be (partly) “caused” by the prior dusk, which would then be before and not after noon—the day is indeed a circular structure. This temporal chicken-and-egg problem is a good reason for challenging the early-bird wisdom. As long as all individuals have similar daily routines (in other words, as long as they are similar chronotypes), the earliest bird has an advantage over anyone getting up later. This was probably true for most preindustrial societies; hence the persistence of the early-bird proverbs. If the distribution of chronotypes becomes as broad as that shown in the first chapter for Central Europe today, then the temporal chicken-and-egg problem starts to apply to the hunt for resources. A small number of very early birds in the population would wake up on their own between four and five in the morning, but even more very late chronotypes would still be awake. There is no reason why these extreme late types couldn’t gather all the mushrooms before the early risers arrived in the forest. They could then go to bed and sell the mushrooms to the early birds in the afternoon. They would even have a monopoly on mushrooms because the next good crop wouldn’t have grown until the next morning. If you have difficulties with the mushroom metaphor, then think of the impact that stock exchanges have on each other. The last stock exchange of the day, Wall Street, influences the first stock exchange of the next day in Tokyo, which in turn will have an impact on all the others between Tokyo and New York.

This myth that early risers are good people and that late risers are lazy has its reasons and merits in rural societies but becomes questionable in a modern 24/7 society. The old moral is so prevalent, however, that it still dominates our beliefs, even in modern times. The postman doesn’t think for a second that the young man might have worked until the early morning hours because he is a night-shift worker or for other reasons. He labels healthy young people who sleep into the day as lazy — as long sleepers. This attitude is reflected in the frequent use of the word-pair early birds and long sleepers (as mentioned by the journalist). Yet this pair is nothing but apples and oranges, because the opposite of early is late and the opposite of long is short. Although duration and timing are the two major qualities of sleep, they are independent from each other. […]

[…] Almost a quarter of the population sleeps around eight hours (averaged over work and free days); close to 60 percent need between 7.5 and 8.5 hours of sleep (the three most populated categories in the graph). People who get by on less than five hours are very rare (but do exist), as are those who need more than ten hours every night. Due to the different sleep needs of individuals, the concept of midsleep was introduced to characterize when people sleep, which also gives us an indication of the relationship between an individual’s internal time and local (external) time. There are just as many short and long sleepers among early chronotypes as there are among late chronotypes; or turned around, there are just as many early and late chronotypes among the short sleepers as there are among the long sleepers.

Thus, the notion that people who get up late sleep longer than others is simply wrong. This judgment presumes that all people go to bed at the same time, which we know isn’t true — certainly not in the world we live in today. But what is it that makes us fall asleep? Is it merely a signal by our biological clock? Surely not, otherwise we couldn’t have an afternoon nap or a siesta. There must be more to falling asleep.

This is an extract from Till Roenneberg's Internal Time published by Harvard University Press