Tired all the time? Your daily coffee habit is likely to blame, say experts – especially if you’re a woman

It’s a daily habit for nearly three-quarters of people in the UK. 

For many, the morning hasn’t begun until that first sip of coffee, after which you’ll feel suitably awake and ready for the day. Or will you?

Experts are increasingly warning that the nation’s caffeine addiction could be partly responsible for a simultaneous energy crisis – with at least a third of Britons suffering exhaustion ‘at least half of the time’, according to polls.

Earlier this year, a major scientific review revealed that even caffeine consumed early in the day could have a detrimental impact on your quality of sleep. 

Using data from brain scans, experts from Wroclaw Medical University in Poland found that the stimulant reduced the amount of restorative deep sleep participants obtained.

This effect, the scientists concluded, explains why coffee drinkers – who consume at least twice the caffeine that’s in tea – may still feel tired, despite getting eight or more hours of sleep.

But caffeine isn’t only responsible for distorted sleep; there are other intriguing reasons why coffee can stunt your energy. 

Here, experts explain all – and reveal if any amount of caffeine is worth drinking.

How much coffee can I drink without getting tired?

As well as impacting sleep, there’s another key reason why caffeine makes you tired in the long run. 

The stimulant results in temporary feelings of alertness due to its impact on a neurotransmitter called adenosine, which helps us feel awake. 

Caffeine blocks the effect of the compound in the brain, interfering with signals that tell us we’re feeling tired. 

However the temporary effect means that, when adenosine builds up again, you’re more likely to notice a sudden bout of tiredness that is often referred to as a ‘caffeine crash’. 

The more coffee you drink – especially repeatedly – the less sensitive you become to adenosine signals, meaning you build up a tolerance. 

This means you need more caffeine to feel the same effect, which is likely to lower sleep quality, exacerbating exhaustion and thus creating a vicious cycle. 

But studies have identified a certain amount of caffeine that’s the least likely to result in this energy-zapping effect. 

Last year, Australian scientists tested the impact of various strengths of coffee, consumed at different points throughout the day, on sleep quality. 

They found that 100mg – equivalent to one filter coffee or a small latte – had no significant impact on sleep even when drunk just 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. 

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.