Caffeine can help with focus and performance — but too much might mess with your sleep, your heart, and your hormones.

Wakes You Up

Caffeine boosts your energy and mood and makes you more alert. That can sometimes be helpful, especially in the morning or when you're trying to work. Though your body doesn't store it, caffeine can affect you for up to 6 hours after you swallow it. But more is not always better. Too much can push you over the line from alert to jittery and anxious.

Interferes With Sleep

Too much caffeine can make it hard to nod off when you go to bed at night. Even moderate amounts can cause insomnia in some people, especially if you have it too close to bedtime. The effects may be worse as you age. Avoid caffeine in the afternoon and evening if you notice it affects your sleep. And remember, it's not just in tea and coffee. It's also in chocolate, energy drinks, and other prepackaged foods and drinks.

Raises Heart Rate

Caffeine is a stimulant and may cause your heart to beat a little faster as it wakes you up. For most folks that's not a problem. But if you have too much caffeine or you're overly sensitive, your pulse may go up too much or stay high too long. It may even feel like your heart beats in a weird rhythm, sometimes called heart palpitations. Some people say it feels as if their ticker skips a beat.

Causes a Headache

If you have caffeine every day, whether in a pill, energy bar, or cup of coffee, you build up tolerance. Then, without your daily dose, you might get a "rebound" headache. It may be worse if you quit caffeine completely and all at once. You'll find your head will feel better if you reduce caffeine a little at a time.

Makes You Pee

Caffeine is a diuretic, which means it can make you pee more. Around 300 milligrams of caffeine -- the amount in three cups of coffee -- is enough to do it if you're not used to it. Water loss is minor and is unlikely to cause dehydration as long as you are otherwise healthy. The diuretic effect can fade if you get the same regular daily dose of caffeine because you build up tolerance.

Boosts Sports Performance

If you do "endurance" sports, like running, biking, or swimming, caffeine might help you go faster and with less muscle pain. It seems to work best in a non-liquid form, like a pill, taken about an hour before you exercise so that your body can absorb it completely. Around 200 to 400 milligrams (2 to 4 cups of coffee) should do it. More than that doesn't seem to help further.

Helps You Recover From a Workout

Some studies show that caffeine can help your body recover more quickly after hard exercise by making and restocking a stored form of fuel called glycogen. It seems to do this best if you combine it with carbohydrates, like in certain sports gels, sports bars, and drinks. Just take care not to overdo the caffeine, which could have the opposite effect on recovery or performance if it interferes with your sleep.

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