It’s two in the afternoon, you can barely keep your eyes on the screen, and you open an energy drink to wake yourself up. For a while it works: your head clears and the work starts moving. Then, a couple of hours later, you catch yourself reading the same line for the third time. You’re more tired than you were before you drank it, and you’re thinking about opening a second one. For a drink with the word “energy” printed on the can, that’s a strange result.
It makes more sense once you know what the caffeine in it does. Caffeine really does wake you up: for a while you feel more awake, and you react and concentrate better. What it leaves untouched is your need for sleep. That points to a reasonable explanation for the slump, and it’s an ordinary one: you were tired before you opened the drink, the afternoon added to it, and at some point the caffeine stopped being enough to hide it. It doesn’t have to be the only thing going on.
The other usual explanations each have something to them, in very different amounts. Sugar may add to the slump, though the evidence for a “sugar crash” is thinner than the name suggests. A tolerance changes things if you have caffeine every day. Dehydration, which shows up on most lists, barely belongs on one. And there is one way an energy drink really can make you more tired: caffeine late in the day can cost you sleep that night, and you may not notice it happening.
If you’ve been wondering whether something is wrong with you, start with these ordinary explanations, because they apply to anyone who drinks energy drinks. This article goes through what’s actually making you tired, what to check in your own day, and what you can change so the energy drink does what you wanted from it, or so you need it less. None of it ends with me telling you to quit. I drink Monsters every day.
What an energy drink actually does
Caffeine is the part of an energy drink that does the work. The taurine, the B vitamins and the ginseng on the label have very little evidence behind them.
The word “energy” on the label is where the confusion starts, because it means two different things. One is energy in the sense of calories, which the sugar in a regular energy drink gives you, the same as food does. The other is feeling awake, and that part is the caffeine. While you’re awake, your brain keeps track of how tired you are, and one of the chemicals it uses to pass the message on is called adenosine. Caffeine fits into the receptors that adenosine uses, and blocks them. Less of the “you’re tired” message gets through, so you feel more awake. How much sleep you need hasn’t changed.
It works a lot like a painkiller on a sprained ankle. The ankle is still sprained; you’ve just stopped hearing from it. When the painkiller stops working and the ankle hurts again, nobody says the painkiller caused the sprain. And like a painkiller, caffeine does real good while it’s working: reaction time, attention and alertness all improve, from doses as small as about 40 mg. It does less for memory and judgment, which is part of why it can’t stand in for sleep. For scale, a small can (250 ml, or 8.4 fl oz) has about 80 mg of caffeine, and a big one (500 ml in Europe, 16 fl oz in the US) typically has around 160, though some go up to about 300.
How long that lasts depends on the dose, on you, and on how much tiredness the caffeine is up against. One experiment shows a small dose against a lot of tiredness. Twelve people who’d slept only five hours were given one small energy drink, with its 80 mg of caffeine, before two hours of dull afternoon driving in a simulator. For about the first 90 minutes they were less sleepy and drifted out of their lane less. A double serving, in an earlier test by the same researchers, helped across the two hours, most clearly in the first.
That isn’t a timetable for your afternoon. It’s what 80 mg did against a five-hour night and a boring drive. What it does show is worth knowing: when those drivers got sleepy again, most of the caffeine would still have been in their blood. Caffeine reaches its highest level within the first couple of hours and then leaves slowly; on average, half of it is still there about five hours after you drank it. So feeling tired again doesn’t mean the caffeine has left your system. Why the lift fades while the caffeine is still there hasn’t been pinned down in a study. The most reasonable explanation I can see is that the tiredness underneath grows past what that amount of caffeine can hide.
Why the crash can leave you more tired than before
If that explanation is right, the extra tiredness has two well-known sources. The first is time awake. The pressure to sleep builds for as long as you’re up, and after a short night you start the day with some of it already there. The second is your body clock. For a lot of people it runs a dip in alertness in the middle of the afternoon, and the dip is built in: it can show up even in people who’ve had no lunch and don’t know what time it is. A big lunch full of carbohydrate makes it deeper.
Either one can leave you more tired at four than you were at two, with or without an energy drink. With one, you’d notice the difference all at once as the lift fades, which is why it can feel as if the drink drained you.
Short nights add up, too, and caffeine seems to do less against them each day. In a sleep lab, 48 people were limited to five hours in bed for five nights in a row. Some got 200 mg of caffeine twice a day, at 8 in the morning and at noon, and the rest got a placebo, a dummy dose with no caffeine in it. For the first three days the caffeine group stayed more alert than the placebo group. By the fourth day there was no difference between them.
So if it’s Thursday, the whole week has been five-hour nights, and the energy drink that worked on Monday does nothing, the brand probably isn’t the problem. There may simply be more tiredness than that much caffeine can hide.
Is it a sugar crash?
Maybe partly. The sugar rush, at least, doesn’t hold up: across 31 experiments on more than 1,200 healthy adults, sugar on average did nothing for mood or alertness, and people felt more tired in the first half hour after it than after a sugar-free drink. Those experiments tested sugar by itself, though. An energy drink is sugar together with caffeine, a combination that has been studied far less, so the research can’t tell you how much of your own slump is the sugar.
You can find out for yourself, cheaply: switch to the zero-sugar version of the same drink for a while and see whether anything changes. Either way, it’s no reason to be afraid of sugar.
Have you built up a tolerance?
If you have caffeine every day, probably, and it changes what an energy drink does for you.
The clearest picture comes from one large experiment. In it, 379 adults went without caffeine overnight, 16 hours in all, and were then given either caffeine or a placebo. Before anyone had swallowed anything, the people who used caffeine regularly were already less alert than the ones who rarely touched it, and had more headaches. On the placebo they got worse as the day went on. On caffeine they came back up to the level of the non-users who’d been given nothing. Not above it.
Read that as someone who has caffeine every day. For the regular users in that experiment, part of what the first caffeine of the day did was undo the hours they’d gone without it, and the same is probably true of your first energy drink or coffee. The name for that is caffeine withdrawal: it typically starts 12 to 24 hours after the last caffeine, and tiredness, headache and trouble concentrating are among its signs. So the foggy feeling before your first caffeine of the day may not be only the morning.
Researchers still argue about how much lift is left on top of that for someone who has caffeine every day, and some reviews find that a real benefit remains. Either way, a dose that once took you from normal to sharp may now go partly toward getting you from below normal back to normal. And if your normal today is tired, because the night was short, then tired is where it leaves you.
If your energy drink is for the gym, the news is better. In a 20-day test of daily caffeine on eleven people, the boost to their cycling power was biggest on the first day and shrank from there, but it was still showing more than two weeks in.
Is it dehydration?
It’s very unlikely to be the reason. Caffeine does make you pee a little more. Across 16 studies, a dose of about 300 mg, roughly what two big cans hold, raised urine output by about 109 ml on average (under 4 fl oz). The two cans that caffeine came in hold almost a liter of liquid, so you end up with more water than you started with.
In people who have caffeine regularly, even that small effect fades: four cups of coffee a day kept 50 regular coffee drinkers as well hydrated as the same amount of water did. A very large dose in one go, around 500 mg or more than three big cans at once, is where the research does start to see an effect.
So drink water because you’re thirsty. It’s unlikely to be what fixes the slump.
How a late energy drink costs you sleep
There is one way an energy drink really can make you more tired. It just does it a day late.
This is the other side of what the drivers showed: the caffeine you’ve stopped feeling is still in you. Half of it is still in your blood about five hours after you drank it, and that’s only an average. For some people it’s an hour and a half, for others nine and a half. Some of the difference is how you’re built and some is circumstance: smoking speeds caffeine up, and pregnancy and the contraceptive pill change it too.
One big can, 160 mg of caffeine, at that average rate.
Opened at 11 AM: about 30 mg still in you at 11 PM.
Opened at 4 PM: about 60 mg at 11 PM.
Opened at 9 PM for a late session: about 120 mg at 11 PM, which is more than a whole small can, drunk at bedtime.
If you clear caffeine slowly, the can you opened at 4 PM leaves about 95 mg at 11 PM instead of 60. These are rough sums from that five-hour figure, to show the shape. They aren’t a measurement of you.
How much leftover is too much? Europe’s food safety agency puts the dose that can disturb sleep in some adults at about 100 mg close to bedtime. The most careful test so far shows how much the size of the dose matters. In it, 23 young men took either 100 mg or 400 mg of caffeine 12, 8 or 4 hours before bed, on different nights, with their sleep recorded at home. On average, the 100 mg dose, a little more than a small can, made no measurable difference to their sleep at any of those times. The 400 mg dose, two and a half big cans’ worth, cost them about 50 minutes of sleep when they took it four hours before bed, and it was still cutting into their deep sleep when they took it twelve hours before.
The part that matters most is what they noticed. With the big dose four hours before bed, they rated their sleep as clearly worse. With the same dose eight or twelve hours before, they rated the quality of their sleep as no worse than usual, while the recordings showed less deep sleep. An older study found the same thing with the same dose at six hours: more than an hour of sleep gone, and no clear difference in the sleep diaries.
On average, across the trials that have tested it, caffeine before sleep costs about 35 to 45 minutes a night. The dose and the timing are the two parts you can do something about.
Put that together and you get a loop that can keep itself going. A late or large dose can cost you sleep without your noticing. You wake up more tired, so the next day you want caffeine sooner, or a second energy drink, and the second one comes later in the day. One review calls it the coffee cycle. In a survey of more than 53,000 students in Norway, the ones who drank energy drinks every day slept about half an hour less a night than the ones who rarely or never did. A survey can’t say which came first, the energy drinks or the short nights. Both can be true at once.
What to check in your own day
You can’t tell the cause from the time the slump arrives. The same slump at four in the afternoon can come from a short night, the afternoon dip, yesterday’s late caffeine, or all three together. What you can do is check a few things, one at a time.
Clues, and what to check
If this is you | What to check |
|---|---|
You’ve been sleeping less than usual, or less than about seven hours most nights | Start here. Caffeine can hide the tiredness from short sleep for a while, and it seems to manage less of that as the short nights add up. |
Your last caffeine of the day comes in the late afternoon or the evening | It may be costing you sleep without your noticing. Have it earlier for a while and see whether your mornings change. |
You have caffeine every day, and you feel foggy or get a headache until the first dose | Part of that first dose may be going toward undoing the hours without caffeine, which leaves less of a lift on top. |
The slump comes at about the same time most afternoons | It may be the afternoon dip that a lot of people have. See whether it shows up on a day without an energy drink too. |
Your energy drink is the full-sugar kind | The sugar may be adding to the slump. Switch to the zero-sugar version of the same drink for a while and see whether anything changes. |
You’re on your second or third energy drink of the day and each one does less | Add up the day’s caffeine in milligrams, coffee included, and look at how you’ve slept this week. |
More than one of these can be true on the same day, and none of them is a diagnosis. And if none of them fits, because you sleep seven hours or more, keep your caffeine modest and early, and still can’t stay awake during the day, take that to a doctor rather than to another energy drink.
What to change
Start with the timing, because changing it costs nothing. Whatever you drink, have the last of it earlier. Our article on how to fix your sleep schedule uses eight hours before bed as the cutoff, which is the figure the US National Institutes of Health give people who are resetting their sleep. For an 11 PM bedtime that means the last energy drink or coffee by about 3 in the afternoon. How strict you need to be depends on how fast you clear caffeine, and you can’t see that from the outside. So treat eight hours as a place to start and judge it by your mornings: if you begin waking up more rested, that’s a sign the late caffeine was costing you sleep.
The evening is the hard case. If the energy drink is for a workout at seven or a session with friends that starts at nine, an early cutoff takes away the one you wanted most. I know this one from the inside: I take too much caffeine too late in the day for my own workouts. A smaller can, or half of a big one, is one way through. In the trial above, 100 mg four hours before bed made no measurable difference to sleep and 400 mg cost fifty minutes of it; a big can sits somewhere between the two, and nobody has tested exactly where. Moving the session earlier is another way, when your week allows it. And some evenings the session simply matters more to you than the sleep. That’s a trade you’re allowed to make, as long as you know you’re making it.
Be careful with the second energy drink, the one you open at four to fix the slump after the first. It will probably lift you for a while. It also adds a fresh dose to the caffeine that’s still in you from the first one, with seven hours to go before bed. That’s how the loop from earlier starts: later caffeine, a shorter night, a more tired tomorrow. If you can, take a short nap instead. Ten or fifteen minutes, not too late in the day, helps almost at once, while a nap that runs past half an hour tends to leave you slow and heavy for a while. And if a nap is impossible where you are, it helps to know that an afternoon dip usually eases without any help, because your body clock pushes harder to keep you awake as the evening comes.
Then the amount. You may need less than you think: the benefits start at about 40 mg, so a small can, or half a big one, is a real dose. At the other end, the guidance for healthy adults comes in two parts: up to about 200 mg at one time, and up to about 400 mg across a whole day, with a lower daily figure in pregnancy. Big cans run from 54 to 328 mg depending on the brand, so one can might be a small part of your day’s caffeine or most of it, and a few are more than the single-dose figure by themselves. The number is printed on most cans, and the coffee, tea and cola you drink count toward the same total.
Then the drink itself. If you want to know whether the sugar is part of your slump, the zero-sugar version takes it out of the picture. It also takes out the calories, 230 of them in a big can of regular Monster. Swapping energy drinks and soft drinks for the zero-calorie versions is one of the changes we make with clients in the second month of coaching. The habit stays as it was; you just pick up a different can.
And then your sleep, because if you’re short on it, that is probably what the caffeine has been hiding, and in the five-night study it hid less with each short night. Adults need seven hours or more, we ask our clients for seven to eight, and if your nights have slid late, the sleep schedule article is the place to start. If what you feel is less tired than flat, with no drive for anything on your list, that’s a slightly different problem, and we wrote about having no motivation to do anything too.
If you drink energy drinks every day and wonder whether you have to give them up: you don’t. Everything above works with them still in your day. If you do decide to cut down, do it in steps, because stopping all at once is what brings on withdrawal: it typically starts 12 to 24 hours after the last caffeine, is at its worst a day or two in, and has usually cleared within nine days. If you’d rather have a coach look at the whole day with you, the caffeine, the sleep and the training around it, that’s what we do at Gains For Gamers.
Caffeine can hide a short night for a while. It can’t replace the sleep.
Questions we get asked
Why do energy drinks make me sleepy right away?
A few things can do it soon after you drink one. If you have caffeine every day, part of what the caffeine does may be to bring you back to your normal, and after a short night your normal is tired. If you’re far enough behind on sleep, there can be more tiredness than that amount of caffeine is able to hide. And if the drink has sugar in it, the sugar may add to it: in experiments that tested sugar by itself, people were on average more tired in the first half hour than after a sugar-free drink.
Is one energy drink a day too much?
It depends on how much caffeine is in it and what else you have that day. Europe’s food safety agency puts its guidance for healthy adults in two parts: up to about 200 mg at one time, and up to about 400 mg across a whole day from everything you drink, with a lower daily figure in pregnancy. The US figure for a day is the same. A big can holds anywhere from 54 to 328 mg, so read the number on yours and count the coffee as well. People also differ a lot in how strongly they react to caffeine and how fast they clear it, so treat those figures as a ceiling, not a target.
Does coffee do the same thing?
Where caffeine is the cause, yes. The caffeine in coffee blocks the same signal, stays in your blood just as long and can cost the same sleep when you have it late, and a large coffee-shop coffee can carry more caffeine than a big energy drink. What coffee usually doesn’t have is the sugar, unless you add it.
Should I wait an hour or two after waking before the first one?
It won’t hurt, but there’s no evidence it prevents an afternoon crash. The idea spread on social media, and when a group of sports nutrition researchers looked for support for it in 2024, they found none. If you slept well, right after waking is when you need caffeine least, which is a fair reason to wait if you like. It isn’t a fix for the slump.
Thanks for reading. I’m Daniel, and I wrote this. I’m a certified personal trainer with more than 10,000 hours on the gym floor with clients, and as founder and head coach of Gains For Gamers, I get to combine the best of my two favorite worlds: gaming and fitness.
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This page is general information from coaches, not medical advice. If you have a medical condition or an injury, or take medication, talk to your doctor before you change your training or diet.