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What are the impacts of caffeine on sleep?

A woman in an orange top yawns while holding a beverage

What’s the science behind caffeine’s effects on sleep – and when should you drink your last coffee of the day?

Caffeine and sleep are awkward bedfellows. The natural stimulant, found in drinks such as coffee and tea, is great at making us feel awake and alert – so it should come as no surprise that caffeine also impairs our ability to nod off and sleep soundly.

With the best coffee machines now offering a higher calibre of brewing at home, many drinkers will feel tempted to keep drinking coffee throughout the day – even after local cafes have closed. Others might rely on high-caffeine energy drinks to stay alert while working late.

For the most part, coffee is good for you – and so are some other caffeinated drinks, including tea. But when your enlivening drink of choice stops you from getting a good night’s sleep, this can lead to adverse effects ranging from short-term issues such as impaired concentration and lowered mood, to the development of chronic health conditions in the long term.

If you want to figure out how to drink caffeine in a sleep-safe manner, it will help to know how exactly this stimulant works in the body, and for how long. We’ve put together this short guide to explain the science of sleep and caffeine – and we’ll conclude by helping you estimate your daily cut-off time for caffeine consumption.

The key facts on caffeine and sleep

  • Caffeine is a stimulant that delays the onset of sleep
  • The drug blocks the action of adenosine, the sleep-inducing neurotransmitter
  • It also promotes the release of adrenaline
  • Caffeine is present in coffee, tea, energy drinks and many soft drinks
  • Surprisingly, some decaf products also contain a significant amount of caffeine
  • It usually takes anywhere from 3-8 hours for the level of caffeine in the body to decrease by half
  • Sleep Doctor advises stopping drinking caffeinated beverages at least eight hours before bedtime

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What are the impacts of caffeine on sleep?

Caffeine has many specific impacts on both the quality and duration of sleep. Researchers have identified effects including:

  • Delayed onset of sleep
  • Increased metabolic rate during sleep
  • Higher incidence of nocturnal awakenings
  • Reduced total nightly sleep duration
  • Increased light (stage 1) sleep
  • REM sleep induced earlier in the night
  • Reduced deep, slow-wave sleep
  • Altered perception of sleep quality

For many people, the most noticeable effect is difficulty in getting to sleep.

There are multiple chemicals in the brain that help us fall asleep. One crucial molecule called adenosine builds up while we’re awake and gradually makes us sleepier. As some sleep experts would say, it causes ‘sleep pressure’.

“High concentrations of adenosine simultaneously turn down the ‘volume’ of wake-promoting regions in the brain and turn up the dial on sleep-inducing regions,” writes Matthew Walker in his book, Why We Sleep.

“When adenosine concentrations peak, an irresistible urge for slumber takes hold. It happens to most people after 12 to 16 hours of being awake.”

One of the reasons caffeine is great at helping us stay awake is that it blocks adenosine receptors in the brain, so the molecule doesn’t take effect in the usual way. Basically, caffeine stops adenosine from shutting down our brains at bedtime.

After the effects of caffeine have worn off, we can suddenly feel very tired – or to put it another way, we ‘crash’. Adenosine keeps building up in the brain while caffeine blocks our adenosine receptors, so we receive a bumper dose of the neurotransmitter when the caffeine wears off.

The release of adrenaline is another key sleep-preventing consequence of consuming caffeine. This hormone can increase our heart rate, breathing rate and level of alertness – all of which work against us when we try to sleep.

Does caffeine make you sleepy?

Many of us drink coffee or other caffeinated drinks when we’re feeling sleepy to begin with. It takes about 30 minutes for the stimulant effects of caffeine to fully develop (some effects are felt after as little as five minutes for some). So it’s possible that, in some cases, we mistake our pre-existing sleepiness for a caffeine-induced effect.

Another reason why some people may feel sleepy after consuming caffeine is the development of caffeine tolerance. Studies have shown that heavy, long-term caffeine consumption may encourage upregulation of adenosine receptors in the brain. With this in mind, people who drink very large quantities of coffee or energy drinks may find themselves consuming greater quantities of caffeine in order to keep experiencing the same effects.

There is some anecdotal evidence that suggests caffeine causes people with ADHD to feel sleepy, possibly due to the release of dopamine. However, there’s not much research to support this theory.

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Can caffeine cause insomnia?

Insomnia is medically defined as the condition of not being able to sleep normally at least three times per week, for a period of three months or longer – even if a person gives themselves the right opportunity to sleep.

When we consume caffeine too late in the day, this artificially impairs our opportunity to sleep. So, while caffeine can make us sleep-deprived, it does not cause insomnia. The same is true of other external factors known to affect sleep quality, such as smoking, alcohol consumption, light and temperature.

The triggers of true insomnia are psychological and biological. These include emotional concerns, anxiety, distress, overactivity of the fight-or-flight nervous system and elevated cortisol levels.

Caffeine is not a root cause of true insomnia – but its effects can make matters worse for people who struggle to sleep.

A worried looking woman lies in bed awake looking at a cup on her bedside table

“People with chronic insomnia may find it helpful to try reducing or eliminating caffeine intake or only consuming it early in the day as strategies for improving sleep,” advises Sleep Foundation.

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How long does caffeine keep you awake?

Caffeine remains active in the body long after consumption. It has a half-life of 3-7 hours, meaning that’s how long it takes for the caffeine in your system to decrease by 50%. Even a coffee, tea or energy drink consumed in the afternoon may affect us at bedtime.

Strongly caffeinated drinks are best drunk early in the day. If you feel the urge to drink tea or coffee after about 3pm, then your smartest bet might be to choose a lower-caffeine or caffeine-free product. (Many ‘decaffeinated’ drinks have significantly reduced caffeine but are not caffeine-free.)

If you don’t want to stop consuming caffeine in the evening, then you could at least cut down. The stimulant effects of caffeine are dose dependent – meaning you can reduce their severity by moderating your caffeine intake.

The amount of caffeine in a cup of coffee varies depending on factors including the serving size, brewing method and the batch of coffee beans used. Here are some ballpark estimates:

  • Brewed coffee (drip coffee): 80-100mg caffeine per 240ml cup
  • Espresso: 65mg caffeine per shot
  • Cold brew: 100mg caffeine per 240ml cup
  • Instant: 60-80mg caffeine per 240ml cup

French press coffee tends to have lower caffeine content than coffee brewed using other popular methods, so if you want to cut down on caffeine, you might want to learn how to use a cafetière.

Energy drinks tend to contain slightly less caffeine than a cup of coffee, while soft drinks and teas usually have lower levels.

As we all know, caffeine has plenty of upsides. It can make us feel alert, has neuroprotective effects and may increase our productivity, to name a few of the positives. We can’t change the reality of caffeine’s adverse effects on sleep – but we can enjoy its good effects with fewer downsides by making smart choices about which caffeinated drinks we consume and when we drink them.

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