Morning Larks and Night Owls: The Impact of Chronotype

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Post by Anastasia Sares

The takeaway

Chronotype, or day/night preference, is a genetically influenced trait that affects how active and alert we are at various points in the day. In the past few years, studies have linked chronotype to both mental and physical health, and people can suffer adverse effects when their daily schedule doesn’t align with their chronotype—a phenomenon called “social jet lag.”

How do we measure chronotype?

The simplest way to measure a person’s chronotype is by asking about their sleeping habits. It’s important to do this for workdays and for free days, as someone might change their habits between the two, and may try to make up for missed sleep on their free days by oversleeping. There are more objective ways to measure chronotype, but they take longer. Actimetry measures the amount of movement a person makes throughout the day by some kind of wearable device. Another popular measure is dim light melatonin onset, or DLMO, which measures how fast people produce the sleep-related hormone melatonin in response to dim light. All of these measures agree with each other pretty well, even an ultra-short version recently developed for use in clinics or as a part of other studies that don’t have much extra time.

Chronotype also varies hugely over the lifespan, with adolescents having very late chronotypes and older adults having earlier chronotypes. People in urban environments have more varied later chronotypes than those in rural environments, and even people’s location within a time zone (eastern vs. western edge) can affect their chronotype due to small differences in sunlight hours.

How is chronotype related to health outcomes?

Late chronotypes (night owls) are prone to more adverse health outcomes like hypertension and depression. However, it is not clear that being a night owl causes these effects. The less our daily schedule is synchronized with our natural sleeping rhythm, the more stress we experience, and this stress is what can lead to health problems. This is called “social jet lag,” and it is more likely to affect night-owls because societal structures tend to follow earlier schedules (the writer of this article, being a moderately late chronotype, still remembers getting up at 6:15 am during high school with much chagrin). Social jet lag is at its most extreme in shift-workers, like hospital staff who have work during the night.

Chronotype isn’t just a matter of psychology; every cell in the body has a circadian rhythm and is affected by these day-night cycles. Under social jet lag, the body’s cellular clocks adjust at different rates and cannot keep up with the switch between workdays and free days. It is these unsynchronized cellular clocks that may be responsible for the health risks of social jet lag. Worldwide, workers’ sleep habits when working from home during the COVID-19 pandemic shifted later, indicating that society as a whole had been under social jet lag.

How can we lessen the impact of social jet lag?

Chronotype can be manipulated to some degree. Being outside during the day can help to naturally regulate your sleep cycle. On the other hand, exposure to light before bed can disturb natural sleep-wake cycles, and so it is helpful to limit your evening screen time if you want to shift your schedule earlier (put away your phone!). A regular sleep schedule will also lower your risk for adverse health effects, especially if your work hours are very early or very late compared to your natural rhythm. However, don’t sacrifice your free-day sleep in order to keep a normal wake-up time. Rather, try to go to sleep at the same time each night.

On a societal level, we can take chronotype into account in school start times and in assigning shifts to workers, something that is already starting to be done. Some researchers are also calling for governments to abolish daylight savings time, which can cause a host of sleep-related problems.

What's the impact?

In our industrialized society, many people live predominantly indoors and are more detached from natural day-night cycles, making their chronotypes later and more varied. It is important to provide daily structures that accommodate differences in chronotype—this will have a significant impact on human health and well-being, as well as increasing work productivity and quality.

References +

  1. Roenneberg, T., Pilz, L. K., Zerbini, G., & Winnebeck, E. C. (2019). Chronotype and social jetlag: A (self-) critical review. Biology, 8(3), 1–19. https://doi.org/10.3390/biology8030054
  2. Shahid, A., Wilkinson, K., Marcu, S., & Shapiro, C. M. (2011). Munich Chronotype Questionnaire (MCTQ). STOP, THAT and One Hundred Other Sleep Scales, 245–247. https://doi.org/10.1007/978-1-4419-9893-4_58
  3. Ghotbi, N., Pilz, L. K., Winnebeck, E. C., Vetter, C., Zerbini, G., Lenssen, D., … Roenneberg, T. (2020). The µMCTQ: An Ultra-Short Version of the Munich ChronoType Questionnaire. Journal of Biological Rhythms, 35(1), 98–110. https://doi.org/10.1177/0748730419886986
  4. Kalmbach, D. A., Schneider, L. D., Cheung, J., Bertrand, S. J., Kariharan, T., Pack, A. I., & Gehrman, P. R. (2017). Genetic Basis of Chronotype in Humans: Insights From Three Landmark GWAS. Sleep, 40(2). https://doi.org/10.1093/sleep/zsw048
  5. Wittmann, M., Dinich, J., Merrow, M., & Roenneberg, T. (2006). Social jetlag: Misalignment of biological and social time. Chronobiology International, 23(1–2), 497–509. https://doi.org/10.1080/07420520500545979
  6. Hulsegge, G., Loef, B., van Kerkhof, L. W., Roenneberg, T., van der Beek, A. J., & Proper, K. I. (2019). Shift work, sleep disturbances and social jetlag in healthcare workers. Journal of Sleep Research, 28(4). https://doi.org/10.1111/jsr.12802
  7. Korman, M., Tkachev, V., Reis, C., Komada, Y., Kitamura, S., Gubin, D., … Roenneberg, T. (2020). COVID-19-mandated social restrictions unveil the impact of social time pressure on sleep and body clock. Scientific Reports, 10(1), 1–10. https://doi.org/10.1038/s41598-020-79299-7
  8. Roenneberg, T., Wirz-Justice, A., Skene, D. J., Ancoli-Israel, S., Wright, K. P., Dijk, D. J., … Klerman, E. B. (2019). Why Should We Abolish Daylight Saving Time? Journal of Biological Rhythms, 34(3), 227–230. https://doi.org/10.1177/0748730419854197