This Is How Long It Takes Before Coffee Stops Working

The sleep debt is real, people.

SourceDennis Welsh/Getty Images

Why you should care

Because sometimes you just need to PTFO.

Hate to break it to you, caffeine junkies: Coffee might feel like the Elixir of Life when you’re sleep-deprived, but research says that eventually the magic fades, and you really should just hit the sack. A study presented at the 2016 meeting of the Associated Professional Sleep Societies held in Denver found that:

After just three nights of bad sleep, caffeine stopped boosting alertness, performance and mood.

The findings further underscore that “sleep debt is real,” says Tracy Doty, the research scientist at Walter Reed Army Institute of Research who led the study. “Every night you’re not sleeping, you’re building up increased sleep debt, and the same amount of caffeine is no longer effective.”

Earlier studies on the effects of caffeine on performance had typically kept subjects awake all night for multiple nights in a row. But Doty wanted to mimic a workweek, restricting sleep to five hours a night (less than the recommended seven to nine hours a night) for five nights. Doty’s team recruited 48 healthy adults who slept 10 hours a night for five nights in the lab to ensure they started at the same baseline level. The following week, the researchers restricted their sleep, flipping the lights on after five hours.

Doty’s team randomly assigned the participants to receive caffeine or a placebo — chewing gum with or without 200 milligrams of caffeine (equivalent to two strong cups of coffee) — twice daily, at 8 a.m. and at noon. From the time they woke up until they went to bed, the participants underwent several rounds of tests that measured their reaction time, mood and sleepiness. “It simulates the workday and even beyond, when you get home and have a million things you have to do,” Doty says.

At first, participants who chewed caffeinated gum showed significant improvements in their reaction-time performance, and they felt happier. But after three nights of restricted sleep, their performance slid back to the levels seen in the placebo group. They were also sleepier and more irritated than those who got the placebo.

Research indicates that sleep debt — the difference between how much sleep you need and how much you get — makes it harder to perform cognitive tasks. Four cups of coffee a day might prevent the decline at first, but as Doty shows, it soon proves no match for sleep debt. “There’s no chemical that can override it,” says Namni Goel, a research associate professor in psychiatry at the University of Pennsylvania’s School of Medicine, who wasn’t involved in the study.

Gettyimages 168834849

Caffeine crystals under a micrograph. Thank you, caffeine.

Source Dr. Jeremy Burgess/Getty Images

Doty points to the so-called adenosine hypothesis as one possible mechanism. As sleep debt builds up, the thinking goes, so does the neurotransmitter adenosine and/or its receptors in the brain. As adenosine binds to its receptors, drowsiness sets in. At first, caffeine might be able to block those receptors, preventing performance declines. But as sleep debt mounts, caffeine fails to keep up with the buildup of adenosine, its receptors or both.

To be sure, Doty’s study looked at only what happens when people consume a constant dose of caffeine at specific time intervals. While the researchers administered caffeine at 8 a.m. and at noon, let’s be real: “A lot of people would have another cup, maybe two or three,” says Goel, who wonders whether increasing caffeine intake would counteract decreases in performance. Doty says she plans to answer this question in follow-up research.

If you anticipate a sleepless workweek, try clocking in 10 hours of sleep a night for a few days beforehand, which will fill your sleep bank enough for you to power through, Doty says. Basically, “there’s no such thing as a free lunch … Eventually, you’ll have to sleep.”

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