Drinking coffee before bed may delay the human circadian rhythm by up to 40 minutes — making it more difficult to go to sleep at night, according to research published today in Science Translational Medicine. It's the first research to show caffeine to have a direct effect on a person's internal timekeeping system. And coffee can even throw off your sleep cycle on evenings following a late-night caffeine boost, the study suggests.

It’s estimated that more than half of American adults over the age of 18 drink coffee every single day. And these coffee drinkers are consuming an average of three cups daily. Numerous studies have examined the effects of coffee’s impacts on the body, due to its high concentration of caffeine. "Caffeine is one of the most widely consumed drugs that’s legal, but is also has these psychoactive impacts," says study author Dr. Tina Burke, a neuroscientist at the University of Colorado at Boulder. However, its effect on our biological clock and internal rhythm hasn’t been well understood.

"Caffeine is one of the most widely consumed drugs that’s legal, but is also has these psychoactive impacts."

The circadian clock is a biological system that sends cues to your body telling you when you should be awake and when you should be asleep. It's controlled by a region of the brain that regulates hormonal and neural activities that trigger certain bodily functions — like sleepiness and alertness — over a 24-hour period. The circadian clock takes cues from a person's environment — that’s why, for instance, your jet lag isn’t permanent. Several things affect these rhythms but "the biggest training cue is light; it's the primary synchronizer for the day," says Burke.

The clock isn't the only thing that makes you sleepy, though. There's also the homeostatic sleep-wake system, which causes the buildup of a chemical called adenosine. Adenosine inhibits processes that make you feel awake. Caffeine blocks the chemical, making you feel more alert.

But Burke and her team wanted to see if caffeine also affects the body's circadian clock, too. So Burke set up two different experiments. The first involved analyzing a group of five volunteers over a 49-day period. The participants took a caffeine pill — equal to the amount of caffeine found in two shots of espresso — every night three hours before bed. This group was compared to people who were exposed to light before bed; it’s well known that too much light can delay a person’s sleep cycle at night. Overall, the caffeine group had their internal rhythms thrown off by 40 minutes — about half the time delay experienced by those who were exposed to light at night.

Then, Burke turned to human cell cultures. The researchers exposed cells associated with the circadian clock to high levels of caffeine in the lab. The caffeine prevented the circadian cells from signaling that it was time for sleep.

Coffee could be putting your entire rhythm out of sync

The research implies that coffee is doing more than just keeping you alert at night. It could be putting your entire rhythm out of sync. That means drinking coffee one night might have effects on your sleep cycle for many nights to come. "You find it difficult to go to sleep at your standard desired time," said Dr. Timothy Roehrs, a researcher of sleep medicine at Henry Ford Hospital in Detroit, who was not involved in the study. Roehrs said this would cause people to be phase delayed, meaning it's difficult to go to sleep during your standard desired time. "If you’re usually going to bed at 11 o’clock, it’s now midnight."

It's also possible that late-night coffee drinking is contributing to sleep issues, such as circadian sleep-wake disorders — where people can't sleep and wake up during the times the want. Because of this, Burke says people need to be much more considerate of the times they drink caffeine.

Though the study links caffeine to changes in the circadian clock, Roehrs said the cellular effects in the lab may not explain the sleep delays seen in the human subjects. "It would be very difficult to figure out," he said. However, he said it makes sense that the drug is directly altering the biological clock. "Here they are showing the inherent characteristics of the cells are being changed," Roehrs said. "It’s a more direct demonstration than what has been seen before."