For almost three in four people in the UK, coffee is part of the daily routine.
For many coffee drinkers, the day barely feels underway until the first cup has been finished. That familiar caffeine boost can bring a sense of alertness—but could it ultimately leave you more tired?
Health experts are increasingly examining whether Britain’s love of caffeine is contributing to a wider energy slump. Polls suggest that at least one in three Britons feels exhausted at least half the time.
A large scientific review published earlier this year also suggested that caffeine may affect sleep quality even when consumed during the morning or early afternoon.
Researchers at Wroclaw Medical University in Poland analysed brain-scan data and found that caffeine reduced the amount of restorative deep sleep participants experienced.
The finding may help explain why regular coffee drinkers—who typically consume at least twice as much caffeine as tea drinkers—can still wake up feeling drained, even after spending eight hours or more in bed.
Research suggests that having a coffee at least four hours before going to bed has little effect on sleep
Sleep disruption is not the only possible explanation for coffee-related fatigue. Caffeine can affect energy levels in several other surprising ways.
Here, specialists outline why coffee may leave you feeling tired later—and explain how much caffeine could be the best choice for protecting your energy and sleep.
How much coffee can I drink without getting tired?
Beyond its effect on sleep, caffeine can also contribute to tiredness over time through the way it interacts with the brain.
The stimulant creates a short-lived feeling of wakefulness by affecting adenosine, a neurotransmitter involved in the body’s sleep and tiredness signals.
Caffeine temporarily blocks adenosine’s action in the brain, disrupting the messages that would otherwise alert us to growing fatigue.
Once the effect fades and adenosine accumulates again, tiredness can arrive suddenly. This familiar drop in energy is often known as a “caffeine crash.”
Frequent coffee consumption can make the brain less responsive to adenosine, particularly when caffeine is consumed repeatedly throughout the day. Over time, this creates tolerance.
As tolerance develops, people may need larger amounts of caffeine to achieve the same boost. That can further interfere with sleep, increase daytime exhaustion and fuel a difficult cycle of tiredness and coffee consumption.
However, research has pointed to a level of caffeine that appears less likely to trigger this energy-draining pattern.
In a study conducted last year, Australian researchers examined how different coffee strengths, consumed at various times, influenced participants’ sleep.
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The researchers found that 100mg of caffeine—the approximate amount in one filter coffee or a small latte—did not significantly affect sleep, even when consumed only four hours before bedtime.
However, the researchers, from Australian Catholic University, added that this varies between people and their study is no guarantee that drinking one coffee a day won’t have an impact on sleep.
Meanwhile, they found that 400mg – the equivalent of four to five espressos – not only stopped people from falling asleep, but also made the rest far less restorative.
Scientists found this to be true even when the last dose was consumed 12 hours before bedtime.
A latte at breakfast could make your exhaustion worse
A common cause of constant exhaustion among women is iron deficiency – estimated to affect about one in three women in their lifetime.
The deficiency happens when the body loses or uses iron faster than it can replace it, and is most commonly triggered by heavy blood loss such as via periods.
Those with conditions such as inflammatory bowel disease and coeliac disease are also more vulnerable.
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Iron is essential for keeping energy levels stable as it plays a key role in transporting oxygen from the lungs to the rest of the body.
It is also vital for helping cells convert food into usable energy.
Although sufficient iron is available through diet – in foods like eggs, red meat and dark green vegetables – some with restricted eating, such as vegans and vegetarians, may also struggle to eat enough.
One Brazilian study estimated that up to half of vegetarian women who menstruate could be iron deficient.
And, according to Toby Richards, professor of surgery at the University of East London, if you’re low on iron, ‘the worst thing you can do is wash your meals down with a coffee.’
Compounds called polyphenols in coffee – as well as in tea – bind to the iron in the gut and block it from being absorbed into the bloodstream.
So, if you drink a cup less than an hour after a meal, much less iron actually gets absorbed. Interestingly, the same effect isn’t seen if you drink the coffee before eating, according to Prof Richards.
One study found that a single cup of coffee reduces iron absorption from a meal by 39 to 90 per cent, depending on the strength of the brew and the type of coffee.
Drinking coffee with meals will not on its own cause an iron deficiency but it can reduce absorption in people who are already lacking in the mineral, Prof Richards stresses.
What’s more, evidence suggests that a milky coffee adds an extra anti-iron element.
The calcium in milk – which is beneficial for growing strong bones – has been proven to reduce the absorption of iron further, for reasons that still aren’t quite understood.
Erin Waks, 25, was shocked that her coffee habit could be worsening her iron deficiency
A study from the nineties published in the American Journal of Nutrition found that giving people an iron-heavy meal alongside anything over 165mg of calcium (the equivalent of half a glass of milk) reduced iron absorption by up to 60 per cent.
However, researchers have stated that this effect is only likely to make a meaningful difference to those who already suffer iron deficiency and, therefore, advise leaving at least an hour between an iron-rich meal and a milky drink.
Someone who is all too familiar with the impact a daily coffee habit can have on energy levels is 25-year-old Erin Deborah Waks, a journalist from London.
Late last year, she began to feel blighted by a bone-deep tiredness that no amount of sleep would solve. She also had a long history of heavy periods.
‘I felt tired and drained all the time, leaving nights out with friends early,’ she said.
‘Last year I ran a half marathon – this year even a 10k would wipe me out for a whole afternoon.’
To combat the exhaustion, she was drinking two cups of coffee a day – once with breakfast and once again with lunch.
But after months of seeing no improvement in her energy levels, she went to her GP.
A blood test revealed her iron levels were low and she was prescribed Ferrograd C, a supplement used to treat iron-deficiency anaemia.
‘The doctor asked me if I drank coffee with meals and explained how it can affect iron absorption. I said I do – often twice a day.
‘It made complete sense – but I was surprised nobody had told me this before, especially as I had been iron deficient in the past.’
Professor Richards believes all women should be screened for iron deficiency every three to five years of their life from the age of 16.
He suggests more frequent testing for higher‑risk groups, including women with a history of iron deficiency or anaemia, vegetarians and women with heavy periods.