Coffee boosts the number of calories people burn during the day but it robs them of vital sleep at night, according to a new study.
Slurping the caffeine-packed beverage every day boosts your exercise by an extra 1,000 steps every 24 hours — linked to better heart health and a longer life.
But it is also detrimental to your sleep, forcing it below the bare minimum of seven hours per night — leaving you less able to concentrate and raising the risk of conditions including Alzheimer’s.
That’s according to research published today in the New England Journal of Medicine that tracked 1,000 coffee drinkers for two weeks. It was led by scientists at the University of California, San Francisco.
Coffee helps someone walk more during the day but is detrimental to their sleep, a study has suggested (stock image)
The above graphs show the amount of sleep someone got while drinking coffee (orange) and not drinking coffee (blue) and steps both on and off coffee
Coffee is perhaps the most commonly consumed beverage in America, being drunk by a whopping three in four people in the country every day.
Some studies have suggested the drink can boost heart health and lower the risk of conditions like diabetes. It may even boost performance in the gym.
These effects appear to be driven by caffeine, which works by blocking receptors in the brain that signal exhaustion allowing someone to remain alert for longer. It also helps to raise heart rate.
But caffeine may also be coffee’s achilles heel because the up to 10 hours it takes to break down means that an afternoon cup of Joe can restrict your sleep.
Everyone needs at least seven hours of sleep a night, according to the Centers for Disease Control and Prevention (CDC), but a third of Americans don’t achieve this.
Failing to get enough has been linked to numerous health problems including difficulty concentrating, memory problems and a higher risk of diseases including Alzheimer’s and obesity.
Some studies have suggested that it is possible to get many of the health benefits of coffee by drinking the decaffeinated version.
Lead study author Dr Gregory Marcus, a cardiologist, said: ‘The reality is that coffee is not all good or all bad — it has different effects.
‘In general, this study suggests that coffee consumption is almost certainly generally safe.
‘But people should recognize that there are these real and measurable physiological effects that could — depending on the individual and their goals of care — be harmful or helpful.’
In the study, researchers recruited 100 healthy men and women in San Francisco who drink coffee. They were about 39 years old.
Each was fitted with a FitBit to track their steps and sleep as well as an electrocardiogram to monitor their heart rate and blood glucose monitors.
They then followed a strict schedule for two weeks.
For two days, they could drink as much caffeinated coffee as they wanted and get the costs reimbursed by the scientists. But for the next two, they needed to abstain. This cycle was put on repeat.
During the study, participants drank about one to three coffees per day on average, although some had as many as six cups.
Results indicated that they got an extra 1,000 steps a day while on coffee, with step counts rising from 9,665 to 10,646 per day on average.
It wasn’t clear why this was the case, although the researchers said it could be linked to feeling like they had more energy or motivation.
They said the uplift in steps was clinically relevant and may have some ‘long-term positive implications’.
On the other hand, however, they also found that participants’ sleep was constricted while drinking coffee.
When not having coffee they slept seven hours and 12 minutes a night on average. But when they did, this fell to six hours and 37 minutes, or down by 35 minutes.
Scientists say that this happens when coffee blocks receptors for the hormone adenosine, which signals drowsiness in the brain, keeping someone awake for longer.
With caffeine taking up to 10 hours to break down in the brain, this means that the effects of a coffee at 2pm may not wear off until midnight.
The study also looked at the impact caffeine had on people’s hearts.
They found that on days where they had more caffeine, they were 50 percent more likely to have premature ventricular contractions — when the ventricles, or lower heart chambers, beat early — and nine percent more likely to have premature atrial contractions — when the atriums, or top heart chambers, beat early.
Experts said this was not dangerous in a healthy individual, but may pose a risk in someone who has a heart problem.
Dr Amit Khera, a cardiologist at the University of Texas who was not involved in the study, told the Washington Post: ‘In healthy people with normal hearts, it’s what I would call a quality-of-life issue, not a life-threatening issue.
‘If you feel your heart is skipping and it bothers you, then based on these study results cutting out coffee could reduce those symptoms.’
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