Caffeine is the most widely consumed drug in the world. The most popular caffeinated beverage in the UK is coffee, which recently overtook tea as the country’s favourite drink, according to a 2023 Statista survey. The British Coffee Association estimates that 95 million cups of coffee are consumed in the UK every single day, with over 80% of the population drinking an average of 2 cups of coffee a day…
So, the question is, is all this coffee good for us? Could caffeine putting our sleep at risk? In this article we look at how much caffeine is in our favourite drinks, and how much is too much.
How does caffeine affect sleep?
Our sleep timing and quality is controlled by 3 systems:
- Circadian rhythms, or body clocks, which thrive on a regular sleep-wake routine Sleep pressure, which increases the longer you’ve been awake, owing to the build up of drowsy-inducing adenosine during waking hours
- The Stress System, which will only switch off to allow recovery when we feel safe and relaxed
Caffeine is a naturally occurring stimulant that acts on the central nervous system to increase alertness. It travels across the blood-brain barrier and blocks receptors in the brain which detect adenosine, the substance that makes us feel sleepy. In this way, caffeine temporarily blocks the effects of sleep pressure.
Caffeine can also promote the ‘fight or flight’ stress system, increasing adrenaline and cortisol, and making it harder to relax.
It typically takes 30-45 minutes for the alerting effects of caffeine in a drink to peak, and the effects typically last 3 to 5 hours. Caffeine consumed too close to bedtime can increase the time taken to fall asleep, and shift us out of deep sleep, and making sleep less restorative. A recent review by Gardiner and colleagues (2023) which combined the results of multiple trials found that on average, caffeine made people:
- take 9 minutes longer to fall asleep
- stay awake for 11 minutes more during the night
- spend 11 minutes less in deep slow wave sleep
- reduce sleep efficiency by 7%
Is there a cut-off time for consuming caffeine before bed?
There is a lot of variation in the amount of time it takes for someone to clear caffeine from their body. It has to be metabolised by the liver and then excreted out via the kidneys. On average, the half-life of caffeine - which means the time taken for half the active substance to be cleared - is 5 hours, but it can range from 2 to 10 hours! If you smoke, your body will typically clear caffeine more quickly, whereas in pregnancy, the effects of caffeine last longer. The effects of caffeine last longer in young children since they cannot metabolise caffeine as quickly as adults.
High doses of caffeine will be slower to clear than smaller doses. A recent review recommended that we allow the following cut-off times to avoid caffeine interfering with sleep:
- For a caffeine shot of around 215mg, designed to be taken by athletes before exercise, allow at least 13.2 hours before bed
- For a cup of coffee, containing around 107mg caffeine per 250ml, allow 8.8 hours before bed
- For a cup of tea, containing around 47mg caffeine per 250ml, there was no recommended cut-off before bed.
The recommendations above suggest that 1pm is a sensible coffee cut-off time for people heading to bed around 10pm. Large energy drinks, such as Monster, contain 150mg caffeine in a 500ml can, so you might want to try an even earlier cut-off
Although the review found that the lower dose of caffeine in tea may not impact on sleep quality, this is an average based on one ‘dose’, and you may find that drinking tea throughout the day can still interfere with the quality of your sleep. You may need to experiment with a cut-off time, to see whether this is true for you.
What are the other effects of caffeine on the brain and body?
Although the wake-promoting effects of caffeine are mostly due to effects on the brain, there are adenosine receptors throughout the body, including on the heart and on smooth muscle cells.
Caffeine also has the following stimulant effects:
- Caffeine promotes the actions of the sympathetic ‘fight or flight’ system, which can cause a slight increase in heart rate and blood pressure, especially when you’re not used to it. However, these effects reduce with habitual use. Caffeine also promotes an increase in breathing efficiency, reaction time and body temperature. These effects can enhance exercise performance in the short term, especially for endurance exercise.
- Caffeine promotes more blood flow to the kidneys, and an increase in urine volume - it acts as a diuretic, making you go to the toilet more frequently.
- Caffeine can increase digestive activity. For some people this is helpful, but for people living with Irritable Bowel Syndrome, it may trigger a flare up.
- Moderate amounts of caffeine (20-200mg) can trigger the release of dopamine in the prefrontal cortex, improving mood and alertness. However higher doses can promote symptoms of anxiety, jitteriness and upset stomach.
How much caffeine is too much?
A review of the effects of caffeine on health concluded that 400mg per day did not give rise to safety concerns. The research behind these numbers is based on body weight – around 6mg/kg per day is deemed safe. Up to 200mg daily is considered safe during pregnancy.
How much is 400mg? It’s around 4 cups of instant coffee or 5 cups of tea. Caffeine is found in most fizzy soft drinks, dark chocolate, energy drinks and sports drinks, and some cold and flu remedies. The amount of caffeine varies by brand, and of course by how strong you make your tea/coffee, and how much you drink.
As a rough guide..
- Can of Monster - 160mg
- Double espresso at a coffee shop - 150mg
- Mug of filter coffee - 100mg
- Mug of instant coffee - 80mg
- 100g bar 70% dark chocolate - 65-80mg
- Mug of black tea - 50-75mg
- Can of coke - 35mg
- Mug of green tea - 35mg
Consuming single doses up to 200mg is unlikely to lead to side effects for most adults. Caffeine is not recommended for toddlers or young children.
Potential side effects from larger doses include:
- Anxiety and jitteriness
- Migraine headaches
- Sleep problems
- Upset stomach
- Increased heart rate and blood pressure.
If you suffer from anxiety, seizures, heart, kidney or liver problems your doctor may advise you to limit your caffeine intake. Your sensitivity to caffeine may change as you get older.
Caffeine masks our true level of sleepiness, which could mean that we underestimate how tired we really are. If the effects of a large amount of caffeine wear off when we’re still awake, all the adenosine which has built up can hit us at full force, causing a caffeine crash.
If you’re sure how much caffeine you’re drinking, you could experiment with a caffeine tracking exercise over the next 24 hours, where you write down all the caffeinate you’re consuming. You may be drinking more caffeine than you realise.
Is caffeine addictive? What are the withdrawal effects?
If you drink coffee every day, you can develop a tolerance, where you have to keep drinking more to have the same alerting effects. This is why you may have started with instant coffee, but then moved to filter coffee, double or even triple espresso to get the same buzz.
Routine consumption of caffeine also leads to dependence, where missing your customary caffeine can actually make you feel more fatigued, and lead to withdrawal headaches, anxiety and palpitations. These symptoms usually begin 12 to 24 hours after the last consumption of caffeine and peak within 1 or 2 days, but they can last for a week.
For this reason, if you decide to cut down on your caffeine consumption, taper down gradually over a week or two. You could start by alternating caffeinated and decaffeinated drinks, or introduce a cut off time which gets gradually earlier in the day. If you do experience withdrawal, you can reverse the effects with caffeine.
Is caffeine good for ADHD?
The most common approach to ADHD medication is to use stimulants to improve focus and concentration. Some studies have tested caffeine as a treatment for ADHD but the results are mixed, and caffeine is not as effective as prescription medications. Experts do not recommend giving caffeine to children. If you are taking medication for ADHD, check with your doctor before taking caffeine, since it could interact with other stimulant drugs.
- Caffeine is found in coffee, tea, green tea, dark chocolate, and some cold and flu remedies.
- In the short term caffeine temporarily boosts alertness, especially when we’re sleep deprived. However, you’re really only borrowing energy from later; when the caffeine clears you can get hit by a ‘caffeine crash’.
- Caffeine can make it harder to fall asleep and stay asleep, and reduce the amount of deep restorative sleep we get.
- It’s recommended to avoid coffee and energy drinks 9 hours before getting into bed to protect your sleep.