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When you’re struggling to finish a term paper, preparing a big presentation or studying for final exams, sleep might be the first thing you sacrifice so you have time to get all the work done. To compensate, you might start pounding coffee to help you study late into the night. But there’s bad news if you’re trying to maintain that kind of schedule for more than a couple days.

New research suggests that when people are getting less sleep for an extended time — about 5 hours of sleep per night — caffeine stops giving them a cognitive boost by the third day.

“We were particularly surprised that the performance advantage conferred by two daily 200-milligram doses of caffeine was lost after three nights of sleep restriction,” lead author Tracy Jill Doty said in a statement. “These results are important, because caffeine is a stimulant widely used to counteract performance decline following periods of restricted sleep. The data from this study suggests that the same effective daily dose of caffeine is not sufficient to prevent performance decline over multiple days of restricted sleep.”

The study: The sample size was fairly small (48 people), so we can’t assume everyone will respond the same way. Those 48 people slept 5 hours per night for five nights straight. They got a caffeine dose equivalent to a large cup of coffee or a placebo around 8 a.m. and another around 12 p.m. each day. For the first two days, the people who got caffeine were performing better on attention and reaction tests. But by the third day, the caffeine was no longer helping them perform any better than the people who didn’t get any caffeine.

However, upping the caffeine dose may have changed that, Doty said.

“We do not know if more caffeine would help, but we do know that the more caffeine you consume, the more negative side-effects you feel (e.g. jitteriness),” Doty said in an email. “This work goes along with lots of other work showing that the negative effects of restricted sleep are difficult to overcome — here this is true even with a hefty daily dose of caffeine.”

The takeaway: So you’re probably better off carefully budgeting your time and making sure you get enough sleep rather than relying on caffeine to power through a frantic week of finals.