If you’re someone who can count the hours of sleep you get on one hand, you're probably always holding a mug of coffee in the other. But that cup of joe might not be helping you as much as you think. According to a new study, caffeine doesn’t seem to improve alertness or performance after three nights of sleeping fewer than five hours.
For the study, published in an online supplement of the journal Sleep, researchers controlled the caffeine intakes and sleep patterns for 48 healthy individuals over five days. Twice a day, each participant was given either 200 mg of caffeine—the amount in a strong cup of coffee—or a placebo. At night, they were only allowed to sleep for five hours. Each hour they were awake, they were given a cognitive task. Researchers also measured their mood, level of sleepiness, and alertness.
At the end of the study, researchers found that caffeine only affected participants' alertness for the first two days. It made no difference for the last three days.
Tracy Jill Doty, lead study author and a research scientist at Walter Reed Army Institute of Research, explained in a statement that, “the same effective daily dose of caffeine is not sufficient to prevent performance decline over multiple days of restricted sleep.”
So if you find yourself up all night with a baby (or annoying neighbors) Sunday and Monday nights, you might try adding another cup Wednesday through Friday to stay alert. But the best way to improve cognitive performance long term? You'll have to try to get some more sleep. (And if you need more reasons to justify a few more hours of shuteye, here’s five).