It’s no secret that drinking coffee shortly before bedtime disrupts sleep, but a new study suggests that caffeine can actually affect our body’s internal clock, pushing back our natural rhythms by nearly an hour.

I’m an avid coffee drinker and a true believer in the positive effects of caffeine, but I impose a strict personal rule when it comes to drinking coffee late in the day: no caffeine after 6:00 pm. Failure to adhere to this rule—regardless of how tired I am—will inevitably prevent me from falling asleep at my usual bedtime. Clearly, the stimulant effects of caffeine have something to do with it, but a research team from the MRC Laboratory of Molecular Biology and the University of Colorado found there’s more to it than that. Their study, which now appears at Science Translational Medicine, shows that caffeine actually affects the body’s circadian rhythm by delaying the release of melatonin in the brain, a hormone that makes us feel sleepy.


To show this, the team recruited five volunteers to determine when melatonin starts to appear in saliva. The participants were required to live in the lab for 49 days without the benefit of a clock or external environmental cues (e.g. sunlight) to inform them of the time.

The volunteers were administered caffeine pills—the equivalent of a double espresso— or a placebo three hours before bedtime. Then, to adjust the body’s internal clock and determine when the release of melatonin occurred, the subjects were exposed to varying levels of light. Dim light was used to indicate nighttime, while bright light was used as a control to delay the human circadian clock.

Participants who took the caffeine pills exhibited a delay in melatonin release by as much as 40 minutes compared to those who took the placebo. The researchers say that caffeine affects adenosine receptors in the brain, which plays an important role in managing a cell’s 24-hour cycle.

Study co-author John O’Neill highlighted the study’s significant in a MRC release:

These findings could have important implications for people with circadian sleep disorders, where their normal 24 hour body clock doesn’t work properly, or even help with getting over jet lag.

Our findings also provide a more complete explanation for why it’s harder for some people to sleep if they’ve had a coffee in the evening—because their internal clockwork thinks that they’re an hour further west. By understanding the effect caffeinated drinks have on our body clock, right down to the level of individual cells, gives greater insight into how we can influence our natural 24 hour cycle—for better or for worse.


Disruptions to the body’s natural rhythms are quite serious, and have been linked to an increase in the risk of certain cancers, heart disease, type 2 diabetes, and Alzheimer’s.

Read the entire study at Science Translational Medicine: “Effects of caffeine on the human circadian clock in vivo and in vitro”.


Email the author at and follow him at @dvorsky. Top image by Pen Waggener/CC BY 2.0

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