Daylight and Caffeine Consumption: Evidence from Time Zone Borders

• In Event: Sugar, Alcohol, and Caffeine: Policy Impacts on Consumption and Health Outcomes

Abstract

Human circadian rhythms are determined by the rhythms of the sun, whereas modern social schedules are dictated by clock time. This mismatch between biological and social time induces fatigue. Caffeine, consumed through coffee, tea, and soft drinks, is the world’s most widely used stimulant for combating fatigue. While prior research has documented an association between fatigue and caffeine use, causal evidence remains limited. 

We exploit quasi-experimental variation in solar time across U.S. time zone borders to estimate the causal effects of daylight schedules on household caffeine purchases. Time zone borders cause a discontinuous shift in clock time, while solar time changes continuously across these borders. This means that on one side of a time zone border, the sun comes up at 7:30am, while just a few miles to the east, it rises at 6:30am by the clock. Since school and work schedules are fixed, this means that people living and working just a few miles away can experience differences in whether the sun is up when they must wake, and how long after sunset they need to go to sleep. Using a geographic regression discontinuity design, we isolate these discontinuous shifts in clock time, and estimate their impact on caffeine purchases. 

Analyzing NielsenIQ Consumer Panel data (2011–2019) comprising 324,884 household-month observations from 10,589 households, we find that employed households on the late-sun rise side of time zone borders purchase 561 mg more caffeine per month as compared to similar households on the early side. This effect is highly seasonal and geographically distinct: during winter, when delayed sunrise is most severe, purchases increase by 696 mg overall, with a larger effect in the North (776 mg) than in the South (703 mg). These findings demonstrate that caffeine consumption is not merely a lifestyle choice, but a behavioral adaptation to the fatigue generated by rigid geographic and institutional time structures.

Author

• Nahae Kang, The Ohio State University
  Presenting Author