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This Is How Long It Takes for Your Body to Start Ignoring Caffeine

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Whether you're pulling late nights at the office or out with your squad, we've all rationalized skimping on sleep with the justification that we'll have an extra shot of espresso in our morning latte to get back on our A game. Hey, we can't always clock a full eight hours. (How Much Coffee is Too Much?) But your trusty shot of caffeine has a shelf life, according to a new study—and making a habit of substituting your morning joe for a good night's rest will become totally ineffective after a certain point.

The study, done by the American Academy of Sleep Medicine, looked at how effective caffeine was after multiple low-sleep (we're talking anything below five hours) nights. (Psst... Are You Sleeping More or Less Than the Average College Student?) The researchers had 48 adults limit themselves to five hours of sleep over the course of five nights; half were given 400mg of caffeine (which is about four cups of coffee) while the other half of participants was given a placebo during the day. The researchers then performed several tests on the participants each day of the five-day study to measure mood, sleepiness, vigilance, and overall ability on standard cognitive tests.

They found that after three nights of not getting enough sleep, caffeine had no effect on the participants' sleep-deprived days—the performance boost you get (especially when it comes to vigilance and motor skills) the first two caffeine-fueled days drops off completely after pulling three late nights in a row.

In other words, you can technically sub coffee for sleep in a pinch, but after three days your body will start ignoring the effects of that espresso. The researchers didn't look at what would happen if you increased your dose of caffeine on the third day, but the bottom line is that relying on artificial energy can't replace the benefits of a good night's rest. (If sleep is out of reach, try on of these 4 Healthy Caffeine Fixes—No Coffee or Soda Required.)