Chronotype and disease: Correlation still doesn't mean causation

[sue_frantz]

One of the perennial challenges in teaching Intro Psych is helping students understand that knowing that two variables are, say, positively correlated does not tell us anything about what causes that relationship. Discussion of this study would work during your coverage of correlations or during your coverage of circadian rhythms as an opportunity to revisit correlations.

The 6.5-year study of 433,268 British adults (Knutson & von Schantz, 2018) provides us with an illustrative example. The researchers asked each person “Do you consider yourself to be definitely a morning person [27%], more a morning than evening person [35%], more an evening than morning person [28%], definitely an evening person [9%].” “Increased eveningness, particularly definite evening type, was associated with increased prevalence of a wide variety of diseases or disorders, including dia- betes, psychological, neurological, respiratory and gastrointestinal/abdominal disorders.”

If your Intro Psych students are like most people, they want to jump to the conclusion that being a night owl will cause a number of health problems that will eventually lead to an early death.

The lead author of this study, Kristen Knutson, thinks the root of the problem is really that of a mismatch. Many of our societies are geared toward the morning chronotype. If you have an evening chronotype but are trying to work, say, 9am to 5pm, you’re fighting against your own internal clock (Khan, 2018).  

Ask students to work in pairs or small groups to generate a list of factors that are associated with good health. Perhaps their list includes things like exercising, getting good sleep, eating well, and developing and maintaining strong social ties. Now ask students to consider why these things may be more difficult for night owls than morning larks. For example, how many recreational sports teams compete at 11pm? How many spin or yoga classes are offered at midnight? How easy is it to find a restaurant with healthy fare at 10:30pm? How hard is it to get enough sleep when you don’t get sleepy until 2am but have to be up at 7am to be a work by 9am? Maintaining social ties is difficult for night owls who want to hang out at 10pm when most of their friends are headed to bed (Clark, 2019).

The cause of the correlation between chronotype and health may be due to this whole host of third factors. What if night owls were allowed to be night owls? Would a night owl yoga instructor, for example, teaching at 11pm have night owl students? Would night owls get more sleep if they worked, say, 2pm to 10pm?

If time allows, ask volunteers to share their chronotypes and how they’ve adjusted their schedules to fit that chronotype. Or, if their schedules don’t fit, to share why that’s a struggle.

In closing this activity, reiterate that knowing that there is a relationship between two variables tells us nothing about why two variables are related. 

References

Clark, B. (2019, May 23). No, night owls aren’t doomed to die early. New York Times. Retrieved from https://www.nytimes.com/2019/05/23/smarter-living/no-night-owls-arent-doomed-to-die-early.html

Khan, A. (2018, April 11). Bad news for night owls. Their risk of early death is 10% higher than for early risers, study finds. Los Angeles Times. Retrieved from https://www.latimes.com/science/sciencenow/la-sci-sn-night-owl-death-20180412-story.html

Knutson, K. L., & von Schantz, M. (2018). Associations between chronotype, morbidity and mortality in the UK Biobank cohort. Chronobiology International, 35(8), 1045–1053. https://doi.org/10.1080/07420528.2018.1454458