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If you’re looking for an extra push during your workout, you might want to be careful about what you’re chugging to get it. Drinking caffeinated energy drinks before working out can make your sleep suffer later on , new research from Spain suggests.

In the study, trained athletes who had avoided caffeine for 48 hours consumed an energy drink containing about 1.3 mg of caffeine per pound of bodyweight 60 minutes before completing a workout. That’s the equivalent of a 180-pound guy consuming 234 mg of caffine, or about one and a half 16-ounce cans of Monster Energy. (While that amount of caffeine may seem high, previous research found no performance effects at a lower dose.) They repeated this one week later, except without the caffeine.   

After drinking the caffeinated energy drink, their self-reported ratings of muscle power were 13 percent higher than they were without the jolt—but they were much more likely to have trouble sleeping afterwards. Nearly one in three caffeine-pounders reported insomnia later that night, a rate three times higher than when their beverages were caffeine-free. They were also more likely to feel nervous, too.

When you take caffeine before exercising, it blocks your receptors for adenosine, a neurotransmitter which sends fatigue signals to your body, says study author Juan Del Coso, Ph.D. It also increases the secretion of the hormones adrenaline and noradrenaline, which spark alertness and contribute to feelings of nervousness.

And ditching the Monster for coffee won’t help you catch some shuteye. It’s the caffeine in these energy drinks that are causing the stimulating effects. So guzzling coffee with a similar dose of caffeine before working out can leave you just as likely to toss and turn in bed later that night, says Del Coso.

If you’re an elite athlete looking for a performance boost or an average guy trying to beat his personal best, you can limit yourself to a max of 1.3 mg of caffeine per pound of bodyweight before exercise—the amount tested in the study. Just know that you’ll probably be suffering minor sleep side effects afterwards, he says.

But the best bet for guys who work out for fun or just to stay in shape is to reduce the amount of caffeine they’re taking in to a level that’ll keep them side-effect free, which varies for each person. One way: Keep a journal of your daily caffeine intake and nightly sleep quality. It’ll help you determine how much caffeine you can tolerate, without leading to sleepless nights.

Want to see how your favorite caffeine sources measure up? Check out the chart below.