Caffeine is the world’s most widely consumed stimulant, valued for its ability to boost alertness, mood, and concentration. It works by blocking adenosine, one of the brain’s key sleep-promoting chemicals. But caffeine’s effects linger far longer than most people realise. Its half-life can range from 2 to 10 hours, depending on the individual, so that the timing and amount of caffeine consumed can significantly influence how easily someone falls asleep, how long they sleep, and how restorative that sleep is.

Despite caffeine’s popularity, surprisingly few studies have examined how everyday caffeine use and sleep influence each other over time, especially in adults with typical working lives. Existing research has mostly focused on adolescents and university students, with findings suggesting a two-way relationship: caffeine can disrupt sleep, and poor sleep can drive people to consume more caffeine the next day. However, these studies rarely consider when caffeine is consumed, nor do they account for real-world factors like work schedules, prior sleep loss, or differences between workdays and days off.

This project examines the two-way relationship between habitual caffeine consumption and sleep health among working adults in New Zealand. Over a 14-day period, participants record their caffeine intake, sleep–wake behaviour, work hours, and daily activities, while wearing an actigraphy device to objectively measure sleep. The study aims to clarify how caffeine dose and timing influence sleep health, and how sleep patterns drive caffeine use. The findings will help build clearer guidance for healthy caffeine habits and better sleep health.