How does caffeine keep you awake, and can you have too much?

Caffeine is a natural ingredient found in coffee beans, tea leaves, cocoa beans and over 50 other plants. Coffee, tea and fizzy drinks are the most common sources of caffeine in our diet, however it can also be found in some medications like cough syrups. Caffeine is a stimulant, when consumed it causes a feeling of alertness which is why so many of us reach for a coffee first thing in the morning and late afternoon to help wake us up and get over that 3pm slump.

But how does caffeine exert its ‘awakening’ effect on us? In order to understand this, we need to understand why we feel tired in the first place. This process is begun by a compound called adenosine, which is naturally released by our bodies throughout the day.

During the course of the day our muscles and brain are actively working. As they do so they trigger the release of adenosine. Once adenosine is released it binds to receptors in our brain, and this binding action promotes muscle relaxation and sleepiness. As the day goes on more adenosine is released, binding to more of these receptors and making us feel progressively more tired.

By the time bedtime rolls around we feel very tired and sleepy due to a whole day’s worth of adenosine binding to those receptors in our brain, telling us we need to sleep. Whilst we’re sleeping our bodies recover from this fatigue by metabolising (getting rid of) adenosine and we wake up feeling refreshed, unless of course you didn’t get enough sleep (less than 6-8 hours), in which case not all the adenosine was metabolised leaving you feeling a little sleepy and feeling the need for a big steaming cup of coffee.

So what effect does caffeine have in this process?

Once caffeine enters our blood stream it heads towards the receptors that adenosine normally binds to and blocks the way, this stops the receptors connecting to those receptors preventing the feeling of sleepiness and influences the release of dopamine, serotonin and adrenalin which all play a role in causing you to feel more alert and awake.

As your body metabolises the caffeine – this could take anything from 3-10 hours – those receptors become vacant again and the adenosine heads straight for them which is why you start to feel sleepy again. However, if you’ve consumed caffeine too close to bed time it takes a while for this sleepy feeling to return which may negatively impact your sleep.

Can you have too much caffeine?

Yes, you can, but too much will be varying amounts of caffeine for different people. Some are more sensitive to caffeine whilst others may have built a tolerance to caffeine after drinking it for long periods of time.

Current guidelines state that adults should consume no more than 400mg per day. During pregnancy, caffeine clearance from the mother’s blood slows down. Therefore pregnant and breastfeeding women should aim for no more than 200mg/day. Excessive intake in pregnant women may result in growth restriction, low birth weight babies or premature labour.

Here are a couple of common drinks and the amount of caffeine they contain per cup:

Tea – 10-50mg

Green tea – 30-50mg

Energy drink – 40-250mg

Single shot espresso – 75-85mg

Instant coffee – 60-100mg

Filter coffee (Short/small) – 157mg

Can of coke – 32-42mg

How do you know if you’d had too much caffeine?

A normal effect of caffeine is to feel slightly more awake and alert, however, if you are particularly sensitive to caffeine or have had too much you might experience side effects such as an upset stomach, headaches, anxiousness, fast heart rate, insomnia or nausea.

If you are currently consuming a large amount of caffeine and wish to cut down, it’s advisable to do so slowly as drastically reducing your caffeine intake can cause withdrawal symptoms such as headaches and drowsiness.  Here are a few tips to help you gradually cut back:

  1. Replace every second cup of coffee with a decaffeinated version;
  2. Swap your coffee for tea, this way you’re still getting some caffeine but not as much;
  3. Reduce the size of your coffee, if you usually order a tall/large simply scaling back to a short/small will half your caffeine intake;
  4. If fizzy drinks like coke are a source of caffeine for you then try cutting back on the amount you have.

If you feel as though you have become tolerant to the effects of caffeine it can help to eliminate caffeine intake for a month to reduce your tolerance.

Caffeine can be part of a healthy balanced lifestyle and has been linked to health benefits such as reduced risk of diabetes, Alzheimer’s and liver cancer as well as improved physical strength and endurance. However, it should not be used in place of sleep, exercise or a healthy diet – using it as an occasional pick-me-up or simply enjoying a few cups of tea is definitely allowed!


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