Debunkin

internal day fit the external one by either compression or expansion. Light and darkness are the most potent of these cues. For most humans, our internal clocks are slightly longer than 24 hours, so they need compression. To achieve this, a body clock exposes more of its internal “day” to light and hides some of its internal “night” in the dark. This results in internal time being slightly later than external time, which is why people with slow internal clocks end up being later chronotypes.

Light, indeed, is the most important external cue to synchronize our internal body clocks, and the lack of light can have severe negative effects on our sleep patterns. Even a well-lit workplace exposes us to no more than 100 Lux, which translates to 1,200 Lux-hours over the course of a 12-hour workday. Meanwhile, on a cloudy day, the intensity of outside light is about 120,000 Lux, which means even a short 20-minute walk outdoors would expose us to 40,000 Lux-hours, or more than thirty-fold the exposure of that entire indoor workday. (Unless you have one of these lamps, which has been the single most important investment in my circadian sanity and general tolerance of dreary New York winters since my meditation practice.)

The detrimental effects of this light deprivation are most pronounced in the elderly and the mentally ill. Many elderly people rarely get a chance to go outside, and the TV is often their primary source of light. (Which leads one to wonder why elderly homes and assisted living facilities aren’t investing in such artificial daylight lamps rather than the countless flatscreen TVs gracing the common areas and even individual rooms in these institutions.)

Internal Time goes on to illuminate many other aspects of how chronotypes and social jet lag impact our daily lives, from birth and suicide rates to when we borrow books from the library to why older men marry younger women, and even why innovators and entrepreneurs tend to have later chronotypes. (One hypothesis: because they were more challenged in school than early types, and always had to invent clever strategies to help them perform despite not being on top of things.)

Roenneberg’s daughter put together this wonderful teaser for her father’s research:

(Thanks, Jalees.)

Ultimately, Roenneberg makes a powerful case against many of the social expectations we have around sleep and productivity:

I am often asked whether we cannot get used to given working hours merely through discipline and by confining our sleep habits to certain times. The assumption inherent in this question is that the human body clock can synchronize to social cues. I tend to find that any such questioner, who usually also displays a somewhat disdainful tone towards the weakness of late chronotypes, is an early type — someone who has never experienced the problems associated with the [desynchronized] sleep-wake behavior of late chronotypes.