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How coffee could be harming your health, from sleep to digestion, according to a dietitian READ MORE: The four blood tests everyone over 40 needs, according to a GP By ZOE HARDY, HEALTH REPORTER Published: 13:52 BST, 25 October 2025 | Updated: 13:54 BST, 25 October 2025 Whether you are a morning person or not, many of us can't start the day without a caffeine fix to fuel our workouts and workdays—with most of us reaching for a coffee within a couple of hours of waking. And whilst coffee has been shown to slash the risk of a number of serious health conditions from cancer to heart disease—for some of us, coffee can have an adverse effect, spiking cortisol and wreaking havoc on our sleep. 'Coffee is one of the most widely enjoyed drinks in the world, and for many of us, that first cup is a cherished ritual,' said Dr Federica Amati, one of the nutritionists behind the ZOE Diet. 'But it's also a powerful physiological stimulant—which can have some surprising effects on our gut and overall health. For lots of people, drinking coffee won't be a problem, but it's not a one-size fits all approach, so listen to your body.' Here, Dr Amati, who is Imperial College London's Nutrition Topic lead, breaks down exactly how coffee could be harming your health, and what changes you can implement to make your morning cup even more enjoyable... Stress and anxiety 'When we drink coffee, especially in the morning, it triggers an acute rise in cortisol—our main stress hormone,' explains Dr Amati. 'For people who are particularly sensitive to caffeine, this can translate into physical feelings of anxiety, such as a racing heart or restlessness.' While coffee can make us feel more ready to face the day by triggering a release of dopamine, drinking it first thing after waking could actually exacerbate a spike in cortisol—when levels of cortisol are naturally high, as a result of the Cortisol Awakening Response (CAR), Dr Amati explains. Whether it's a flat white, cortado or double espresso, many of us can't start the day without coffee 'Genetics play an important role here too,' she explained. Whilst some people can metabolise caffeine relatively quickly—meaning they can tolerate more coffee without feeling anxious or jittery—others remain sensitive to its effects for longer. She added: 'These effects are most pronounced in people who don't drink coffee regularly, and tend to lessen with habitual use as the body develops tolerance. 'This isn't a problem for everyone, so listen to your body to work out what's best for you.' How to avoid the cortisol spike 'To minimise this cortisol spike, it's best to wait until after your natural morning cortisol peak has passed—typically half-an-hour to two hours after waking. 'Mid-to-late morning is usually the most supportive time for that first cup,' Dr Amati added. It can interrupt your sleep Although coffee can help boost alertness in the morning, it can disturb sleep if consumed too close to bedtime—particularly for individuals who are more sensitive to caffeine. A study published in Communications Biology found that drinking just two cups of coffee after midday, around 200mg of caffeine, could be enough to shift the brain towards a more excitable and dynamic state during the night, wreaking havoc on your energy levels. The researchers also noted that certain brain signals were flatter, suggesting that, under the influence of caffeine, the brain more closely resembles a 'critical state'––where we are most responsive and alert, potentially disrupting repair and recovery. Coffee has been found to knock out peoples deep sleep, with those who drink it too close to bed experiencing fewer delta brain waves, linked to deep sleep, and more beta waves associated with wakefulness 'Caffeine acts by blocking adenosine receptors in the brain—the same receptors that promote sleepiness—which is why it can delay sleep onset and reduce sleep quality if consumed too late in the day,' Dr Amati explained. 'Many people find that drinking coffee within six hours of bedtime disrupts their sleep, so keeping caffeine to the morning or early afternoon is best for restorative rest.' Deep sleep has long been shown to protect against dementia, facilitating the removal of waste products from the brain. Caffeine's half-life is estimated to be around six hours, meaning that half the caffeine from a 2pm coffee is still in our systems at 8pm. This is why late afternoon cups of coffee, whilst helpful for boosting alertness towards the end of the workday, can disrupt sleep. It may block iron absorption 'Coffee's polyphenols—the same antioxidant linked to many of its health benefits—can slightly reduce the absorption of certain minerals like iron,' Dr Amati explained. Some studies have found that just one cup with an iron-rich meal can reduce absorption by up to 40 per cent. This is partly due to caffeine's diuretic properties, meaning that it slightly increases the expulsion of water-soluble minerals and vitamins, including vitamin C, calcium and magnesium. But, Dr Amati said, 'this effect is small and unlikely to cause deficiencies in the context of a balanced diet.' Dr Federica Amati said whilst coffee has many health benefits, for people who are particularly sensitive to caffeine, it can cause physical feelings of anxiety such as a racing heart or restlessness To help limit these effects, Dr Amati recommends increasing iron intake through dietary sources such as red meat and pulses. Waiting at least an hour after drinking coffee before eating can also help minimise the loss of vitamins and minerals. It stimulates your digestion—which isn't always a good thing 'Coffee also stimulates the digestive system, increasing stomach acid and gut motility,' says Dr Amati. 'While this can support bowel regularity for some, it can also lead to heartburn, stomach cramps, or worsening of irritable bowel symptoms in others.' This is because caffeine stimulates the production of stomach acid and relaxes the valve at the top of the stomach, increasing the risk of stomach acid escaping back up the oesophagus, causing a burning sensation. Research has long shown caffeine stimulates the production of the hormone gastrin, which is responsible for our gastric acid production. Although this helps kick start the process of digestion, it also adds acidity to the stomach leading to discomfort. How to avoid heartburn and acid reflux To prevent acid reflux, doctors advise waiting at least an hour after waking before reaching for a coffee as this should give the body enough time to regulate the hormones responsible for digestion. If the issue persists, Dr Amati recommends monitoring your symptoms and caffeine intake to see if coffee is the trigger. It can ramp up blood pressure and cholesterol Many types of coffee will contain natural compounds, like diterpenes, which have been linked to higher levels of LDL cholesterol which can block arteries and increase the risk of heart complications. And as Dr Amati explains, some brewing methods can be more harmful than others. 'Unfiltered coffee, such as French press or Turkish coffee, contains compounds called cafestol and kahweol, which can raise LDL (the so-called ‘bad’) cholesterol,' Dr Amati explains. 'Filtered coffee is therefore a better choice for people concerned about cholesterol levels, including those with diabetes or who smoke.' Caffeinated or decaf? 'Some people should be particularly cautious with caffeine — including pregnant women, those with anxiety disorders, heart rhythm abnormalities, or uncontrolled high blood pressure, as well as children,' Dr Amati warned. Coffee¿can slash the risk of a heart attack and stroke by a third, scientists say - but only if you drink it at specific time of day 'Preparation also matters: black coffee is generally best for health, while adding sugar and cream can offset many of its potential benefits,' she added. 'But interestingly, both caffeinated and decaffeinated coffee offer similar protective effects against several chronic diseases, suggesting that coffee’s health benefits extend beyond caffeine itself.' How to cut back 'For those looking to reduce their intake, a gradual tapering approach helps minimise withdrawal symptoms such as headaches or fatigue,' Dr Amati advised. Pushing back your morning coffee until after breakfast, once your natural cortisol peak has levelled out, can also help reduce cravings. Overall, moderate coffee consumption — around three to five cups per day, providing up to 400 mg of caffeine for most adults — is considered part of a healthy diet. Share or comment on this article: How coffee could be harming your health, from sleep to digestion, according to a dietitian Add comment
