It is well known that drinking coffee at night can keep people awake, but scientists from the Medical Research Council Laboratory of Molecular Biology in the UK and the University of Colorado have made a discovery that may hold the key to how caffeine consumption affects the underlying body clock.

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Coffee may set the body clock back by 40 minutes, researchers find.

The body clock, or circadian rhythm, operates in every single cell in the body, turning genes on and off at different times to allow us to adapt to the external cycle of night and day. The hormone melatonin, which makes people feel sleepy, is released upon certain triggers, such as the dimming of light.

Disruption of the rhythm, through shift work, jet lag or sleep disorders, can lead to a range of sleep conditions, with knock-on effects such as heart disease, type 2 diabetes, Alzheimer's disease and certain psychiatric conditions.

It now appears that drinking the equivalent of a double espresso 3 hours before going to sleep can turn back the clock by an hour, by delaying a rise in the level of the hormone melatonin.

The finding could have important implications for a range of sleep conditions at a time when more than 25% of the US population are not getting enough sleep, while nearly 10% experience chronic insomnia, according to the Centers for Disease Control and Prevention (CDC).

Fast facts about coffee
  • 61% of Americans drink coffee every day
  • 41% drink more coffee than soft drinks
  • 25-39-year-olds are more likely to drink coffee than other groups.

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The report, published in Science Translational Medicine, describes an experiment in which five people were invited to live in a lab for 49 days without a clock or any knowledge of external light to tell them if it was night or day.

Participants were exposed to bright or dim light; bright light, like caffeine, is known to be a stimulus that lengthens the circadian phase.

Caffeine found to delay melatonin release

The participants were given either caffeine - the equivalent of a double espresso - or a placebo 3 hours before going to sleep. Their saliva was then tested 3 hours later to see how much melatonin had been produced.

When caffeine was given, melatonin levels rose around 40 minutes later compared with the placebo. This represented a shift about half as long as that caused by bright light.

To confirm the results, a team of UK-based researchers added caffeine to human cells in a lab, and found that there, too, the built-in circadian clock was delayed.

The results showed that caffeine acts directly on the adenosine receptors that are found in all cells. In this way, it increases the levels of cyclic adenosine monophosphate (cAMP) - an important intracellular messenger molecule.

The cAMP molecule plays an important role in the inner workings of the body clock, hence the conclusion that caffeine's biochemical effects and its action in delaying the circadian rhythm are linked. Reducing the levels of the cAMP protein on the cell surface, moreover, leads to a decrease in the delay that caffeine would normally produce.

Commenting on the findings, lead researcher Dr. John O'Neill, of the Medical Research Council (MRC) Laboratory of Molecular Biology, says:

"These findings 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."

He adds that the findings, "right down to the level of individual cells," can help us understand how the natural 24-hour clock can be influenced, "for better or for worse."

Comparing jet lag with caffeine consumption, he describes it as if people's "internal clockwork thinks that they're an hour further west." He adds that knowing how caffeine affects the internal workings of the body clock could be useful in treating jet lag brought on by international travel.

Earlier this year, Medical News Today ran a Spotlight feature looking at how caffeine affects our health.