For almost three-quarters of people in the UK, coffee is part of the daily routine.
For many, the day does not truly start until they have taken their first sip and feel more alert and prepared to face the hours ahead. But does coffee really provide lasting energy?
Experts are increasingly warning that the nation’s reliance on caffeine may be contributing to a wider energy crisis. Polls suggest at least one in three Britons feels exhausted “at least half of the time”.
Earlier this year, a major scientific review found that caffeine consumed in the morning may still affect sleep quality later that night.
Researchers at Wroclaw Medical University in Poland used brain-scan data to show that the stimulant reduced the amount of restorative deep sleep participants experienced.
The scientists said this could help explain why coffee drinkers—who typically consume at least twice as much caffeine as tea drinkers—may continue to feel tired even after sleeping for eight hours or more.
Research suggests that drinking one coffee at least four hours before bed has little effect on sleep
Poor sleep is not the only way caffeine can leave you feeling drained. Coffee may also affect energy levels through several other mechanisms.
Here, experts explain why caffeine can contribute to tiredness over time—and whether any amount may be worth drinking.
How much coffee can I drink without getting tired?
Alongside its potential impact on sleep, caffeine can make you feel more tired in the long term for another important reason.
The stimulant creates a temporary sense of alertness by affecting adenosine, a neurotransmitter involved in signalling tiredness.
Caffeine blocks adenosine’s effects in the brain, disrupting the signals that would otherwise tell you that you are becoming tired.
That boost does not last forever. When adenosine builds up again, you may experience a sudden wave of fatigue commonly known as a “caffeine crash”.
Drinking coffee more frequently, particularly several times a day, can also make the brain less responsive to adenosine signals. Over time, this leads to greater caffeine tolerance.
As tolerance develops, you may need more caffeine to achieve the same alertness. That can further undermine sleep quality, intensify exhaustion and create a self-perpetuating cycle of tiredness.
However, research has pointed to a level of caffeine that appears least likely to trigger this energy-sapping effect.
Last year, Australian researchers examined how different amounts of coffee, consumed at various times of day, affected sleep quality.
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They found that 100mg of caffeine—the amount in one filter coffee or a small latte—had no significant effect on 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.
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.
