Energy drinks and sleep: how long does caffeine last?

Plenty of people assume an energy drink works for as long as they can feel the lift.

You drink one, you are more alert for an hour or two, the effect wears off, and in the evening you go to bed as usual.

Caffeine does not quite work like that.

Just because you no longer feel particularly wired does not mean a significant amount is not still in your system. And that can make your sleep worse even if you fall asleep fairly easily.

What happens is not necessarily lying awake all night.

It may be that:

• you fall asleep somewhat later,
• you wake more often,
• your sleep is lighter,
• your night is shorter,
• and you feel less rested in the morning.

So the next day you reach for an energy drink again.

How does caffeine keep you awake?

Over the course of the day a substance called adenosine gradually builds up in the body. Adenosine plays a role in making you sleepy: generally, the longer you have been awake, the stronger the drive to sleep becomes.

Caffeine does not remove that tiredness.

It temporarily blocks the receptors through which adenosine acts. That makes you feel less sleepy, while your body may in fact still be just as tired.

This is an important distinction.

An energy drink does not give you back two hours of lost sleep. At best it temporarily reduces your sense of how tired you are.

As the caffeine wears off, the sleepiness it had been masking can return. That is one reason people suddenly feel much more tired a few hours after an energy drink.

How long does it take to work?

Caffeine generally does not act instantly, although with an energy drink the cold, the sweet taste and the fizz alone can give a quick, refreshing sensation.

Caffeine can start having a noticeable effect within roughly 15–45 minutes, with blood concentrations typically peaking somewhat later.

That is why you can drink an energy drink and feel more alert shortly afterwards.

The problem is more that it clears much more slowly than it starts working.

What does caffeine’s half-life mean?

Caffeine has an average half-life of roughly 4–6 hours in adults, though differences between people can be considerable. The half-life is how long it takes for the amount of caffeine in your body to fall by half.

Say you drink a 500 ml energy drink at 4pm containing roughly 160 mg of caffeine.

If your caffeine half-life is five hours, then approximately:

• at 4pm: 160 mg,
• at 9pm: 80 mg,
• at 2am: 40 mg,
• at 7am: roughly 20 mg may still be in your system.

This is a simplified example, of course. Caffeine is not broken down at the same rate in everyone.

Why does it affect everyone differently?

You almost certainly know someone who drinks a coffee in the evening and falls asleep ten minutes later.

It is easy to conclude from that that caffeine has no effect on them.

In reality, many things can influence how quickly caffeine is broken down:

• genetics,
• age,
• regular caffeine intake,
• smoking,
• pregnancy,
• hormonal contraceptives,
• certain medications,
• some liver conditions,
• the amount consumed.

Research indicates individual differences can be substantial, so the same amount at the same time can affect two people’s sleep entirely differently.

For some people an afternoon energy drink matters little by the evening. For others even an early-afternoon dose noticeably worsens their sleep.

That is why there is no precise cut-off time that is reliably right for everyone.

How long before bed should you stop?

As a general starting point, it is worth avoiding larger doses of caffeine within at least 6 hours of bedtime.

In one well-known study, 400 mg of caffeine still disrupted sleep even when participants took it six hours before bed.

That does not mean a single smaller energy drink will definitely ruin everyone’s night.

The 400 mg used in the study is a fairly large dose — roughly the caffeine in two and a half average half-litre energy drinks.

The lesson is rather that caffeine can last considerably longer than what we directly feel.

The CDC’s general sleep advice is to avoid caffeine in the afternoon and evening where possible. Another CDC source suggests a caffeine-free window of at least five hours before bed, and more if you are sensitive.

In practice, a good starting rule can be:

• if you go to bed at 10pm, no energy drinks after 4pm;
• if you are sensitive to caffeine, try stopping after lunch;
• if you still sleep badly, move your last caffeinated drink earlier still.

You do not have to set a permanent rule straight away.

Try having no caffeine after 2pm for a week, then see whether these change:

• how long it takes you to fall asleep,
• how many times you wake at night,
• how you feel in the morning,
• how much you want an energy drink the next day.

Even 100 mg of caffeine can matter

Many people assume only very high amounts of caffeine cause sleep problems.

According to EFSA, however, a single 100 mg dose of caffeine may affect the duration and pattern of some adults’ sleep, particularly when consumed close to bedtime.

Roughly 100 mg of caffeine can mean:

• a 330 ml average energy drink,
• a stronger coffee,
• a smaller energy drink plus a glass of cola,
• or several glasses of caffeinated soft drinks over the afternoon and evening.

So you do not necessarily have to drink a half-litre energy drink in the evening for caffeine to affect your sleep.

The amounts you have across the whole day add up.

It is not only the energy drinks

You may have stopped drinking energy drinks in the afternoon while still taking caffeine from other sources.

That includes, for example:

• coffee,
• cola,
• black tea,
• green tea,
• iced tea,
• pre-workout,
• caffeine tablets,
• some headache medicines,
• caffeinated water flavourings and supplements.

As far as the body is concerned, there is little difference between caffeine occurring naturally in coffee and caffeine added to an energy drink. Your daily total has to be counted across every source.

Here is a completely ordinary day:

• a morning coffee: roughly 80–100 mg,
• a 500 ml energy drink mid-morning: roughly 160 mg,
• a can of cola with lunch: roughly 30–40 mg,
• another coffee in the afternoon: another 80–100 mg.

That comes to roughly 350–400 mg of caffeine.

You may have had the energy drink in the morning, but the afternoon coffee and cola keep your caffeine level topped up.

Falling asleep does not mean your sleep is unaffected

This is one of the most common misunderstandings.

I fall asleep fine even after an energy drink.

That alone does not prove caffeine is not affecting you.

Sleep is not only about whether you managed to fall asleep.

What also matters is:

• how long you sleep in total,
• how often you wake up,
• how much time you spend in deeper sleep stages,
• how fragmented your night is,
• how rested you feel in the morning.

Caffeine can reduce total sleep time, worsen sleep efficiency and make sleep lighter. How strong the effect is depends on the dose, the timing and individual sensitivity.

So you may fall asleep quickly in the evening and still:

• wake earlier,
• wake several times in the night,
• feel duller in the morning,
• or feel as though seven or eight hours of sleep did nothing for you.

In that case it may well be that caffeine did not affect falling asleep, but the quality of the sleep itself.

Why do we not always notice?

People do not necessarily judge their own sleep quality accurately.

You may wake briefly a few times in the night without remembering it in the morning. You may also have got used to being tired in the morning, so that has become your normal.

And if you have caffeine first thing every morning, you quickly mask the consequences of the worse sleep.

That makes the connection hard to spot:

1. you drink an energy drink in the afternoon,
2. you fall asleep in the evening,
3. your sleep is nevertheless less restful,
4. you are tired in the morning,
5. you have another energy drink,
6. so you feel functional again during the day.

From the outside it looks as though the energy drink helps your tiredness.

Meanwhile it may be partly sustaining it.

What is the caffeine–sleep-debt loop?

After a bad night it is entirely natural to want caffeine.

The trouble starts when you keep trying to maintain your alertness with caffeine later in the day too.

The loop can look like this:

1. you sleep too little or badly,
2. you have an energy drink in the morning,
3. you get tired again in the afternoon,
4. you have another energy drink or a coffee,
5. you sleep later or worse that evening,
6. you wake even more tired the next day,
7. so you need even more caffeine.

After a while it becomes hard to say whether the original tiredness caused the energy drink habit, or the energy drink habit is sustaining the bad sleep.

Often both are true at once.

Why can you wake up tired after a full night?

Sleep duration and sleep quality are not the same thing.

You can spend eight hours in bed while during the night:

• you wake several times,
• you spend a lot of time in lighter sleep,
• you have more trouble reaching deeper sleep,
• you wake earlier than usual.

One possible effect of caffeine is that sleep becomes less restful.

An evening energy drink can also spoil the night in other ways:

• a large volume of fluid means a trip to the bathroom,
• the sugary version raises your evening energy intake,
• the fizz or acidity can cause stomach discomfort,
• you may feel palpitations or restlessness.

So it is not always only about caffeine’s direct stimulant effect.

Can a sugar-free energy drink also worsen sleep?

Yes.

A sugar-free or zero energy drink may contain less sugar and fewer calories, but its caffeine content is often identical to the regular version.

So if your problem is that:

• you have trouble falling asleep,
• you wake often,
• you are tired in the morning,
• or you feel more restless in the evening,

then switching from regular to zero probably will not solve the whole problem.

Cutting the sugar can still be worthwhile, but where sleep is concerned it is primarily the amount and timing of caffeine that count.

Evening cola counts too

Many people do not treat cola as a caffeinated drink, yet most regular and many sugar-free versions contain caffeine.

A cola of roughly 355 ml can contain around 40 mg of caffeine.

Compared with an energy drink that does not sound like much.

But if you drink half a litre to a litre of cola in the evening, that is around 50–60 mg of caffeine — on top of the energy drink and coffee you had earlier in the day.

It is especially easy to overlook if you have several glasses after dinner or while watching something.

If you sleep badly, it is worth trying caffeine-free cola in the evening.

Watch out that “zero” is not the same as “caffeine-free”. Always look for the exact information on the label.

What about green and black tea?

Black and green tea contain caffeine too.

Generally less than a large energy drink or a strong coffee, but depending on how it is made and how much you drink it can still be significant.

If you drink tea in the evening, better choices are:

• fruit tea,
• rooibos,
• decaffeinated tea,
• or another naturally caffeine-free herbal tea.

The word “tea” on its own does not mean the drink is caffeine-free.

Does being used to caffeine help?

Tolerance can develop with regular intake.

That means the same amount of caffeine produces a less noticeable effect over time. You may no longer feel visibly alert or restless after an energy drink.

The effect on sleep, however, does not necessarily disappear entirely.

You may subjectively feel that caffeine barely affects you, while your sleep is still shorter or lighter.

Tolerance can also mean you need a larger dose for the same sense of alertness. That increases the chance of caffeine still being in your system late in the evening.

What to do if you always crash in the afternoon

The first reaction is often that you need another energy drink.

It is worth going through a few simple things first:

• Did you eat a proper lunch?
• Have you drunk enough?
• How many hours did you sleep?
• Have you been sitting in one place for hours?
• Was lunch too large and heavy?
• Has your morning caffeine just worn off?
• Are you feeling withdrawal before the next dose?

Try this order:

1. drink a large glass of water,
2. eat something if you have not eaten for hours,
3. walk for five to ten minutes,
4. wait another ten minutes,
5. and only then decide about caffeine.

If you do choose caffeine, a smaller dose earlier in the day generally makes it easier to limit the evening effect.

What to drink in the evening instead

Choose a drink that not only lacks the caffeine but also keeps something of what made the energy drink satisfying.

For example:

• ice-cold sparkling water,
• caffeine-free cola,
• caffeine-free iced tea,
• fruit tea,
• sugar-free lemonade,
• decaf coffee,
• flavoured water,
• a caffeine-free soft drink.

If what you mostly miss is the fizz and the cold, a glass of plain water probably will not be an appealing enough alternative.

There is no need to force the healthiest-looking drink.

A good substitute is one you genuinely choose instead of the evening energy drink.

A simple experiment to test it on yourself

You do not have to give up all afternoon caffeine for good straight away.

Try this for a week:

1. Note the time of your last caffeine

Each day, write down when you last had an energy drink, coffee, cola or tea.

2. Draw a line

To begin with, no caffeine within at least six hours of bedtime.

If you go to bed at 10pm, choose a caffeine-free drink after 4pm.

3. Track your sleep

In the morning, note:

• how long it took you to fall asleep,
• how many times you woke,
• how many hours you slept,
• how rested you feel on a scale of ten,
• how much you want an energy drink that morning.

4. Move it earlier if needed

If nothing changes after a week, try moving your last caffeine to around lunchtime.

This will tell you far more about your own sensitivity than any general rule.

When should you see a doctor?

Not every sleep problem is caused by energy drinks.

It is worth talking to a healthcare professional if:

• you regularly struggle to fall asleep,
• you often wake during the night,
• you are constantly exhausted despite adequate sleep time,
• you snore loudly or someone notices you stop breathing,
• you fall asleep involuntarily during the day,
• you get a rapid or irregular heartbeat,
• you get chest pain,
• or the sleep problem persists for several weeks.

The CDC also recommends seeing a healthcare professional for persistent sleep difficulties or symptoms suggesting a sleep disorder.

Cutting down on energy drinks can help a great deal, but it is no substitute for investigating a possible sleep disorder.

In short

Caffeine can last considerably longer than the direct lift you feel from it.

Its average half-life is roughly 4–6 hours, so a significant part of the caffeine from an afternoon energy drink can still be in your system at bedtime.

As little as 100 mg of caffeine may affect some people’s sleep, particularly when consumed close to bedtime.

And even if you can fall asleep, your sleep may still be shorter, lighter or less restful.

As a start, it is worth:

• stopping caffeine at least six hours before bed,
• counting coffee, cola and tea as well,
• choosing a caffeine-free alternative in the evening,
• and tracking how you feel in the mornings for a week or two.

It may not be the energy drink that gets you through tired days.

It may be part of the reason you keep needing one.