A rigorous new study that examined the health effects of coffee consumption found good news and bad news for coffee lovers.

The research showed that coffee has striking effects on physical activity levels, causing people to move more, taking, on average, 1,000 extra steps a day – a significant boost in activity that might help explain why coffee consumption has long been linked to better health.

But the study, published in the New England Journal of Medicine, did find some downsides to a daily cuppa. It showed that people lost about 36 minutes of nightly sleep on days when they drank coffee – and the more coffee they drank, the less they slept.

The research also looked at coffee’s effect on heart palpitations, a relatively common experience for healthy coffee drinkers. The study found that in healthy men and women, coffee did not cause a common type of palpitation known as premature atrial contractions, even though some health authorities have warned that this could be a side effect of drinking coffee.

But coffee consumption can lead to an increase in another type of heart palpitation, known as premature ventricular contractions. These extra or irregular heartbeats are fairly common and benign. Almost everyone experiences them on occasion, and while they can be unnerving, most experts say they’re not usually a cause for concern in healthy people.

The findings suggest that the health effects of coffee are complex. While coffee is beneficial for many people and can lower the risk of chronic diseases and perhaps even extend your life span, it can also disrupt your sleep and may cause some heart palpitations.

“The reality is that coffee is not all good or all bad – it has different effects,” said Gregory M. Marcus, an author of the study and a professor of medicine in the division of cardiology at the University of California at San Francisco. “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.”

OVERALL HEALTH EFFECTS

Coffee is among the world’s most commonly consumed beverages, and decades of research suggest that it has mostly beneficial effects. Many observational studies show that coffee drinkers live longer and have lower rates of diabetes, cancer, liver disease, depression and other chronic conditions. But much of the data comes from large epidemiological studies, which show only correlations, not cause and effect. They also rely on self-reported data, which is not always reliable.

At the same time, the research on coffee and cardiovascular health has been somewhat conflicting. Early studies indicated that coffee might be detrimental to the heart because it spikes blood pressure, heart rate and adrenaline, and increases cholesterol levels.

More recent studies have found that drinking several cups of coffee daily – including decaffeinated coffee – could actually lower the risk of dying from heart disease or a stroke, which some experts attribute to the large amounts of antioxidants and other anti-inflammatory compounds in coffee.

Despite a lack of strong evidence, health authorities have often warned people with heart conditions, particularly those with heart rhythm disorders such as atrial fibrillation, to avoid coffee and other caffeinated beverages out of concern that they might trigger palpitations.

To get a clearer sense of coffee’s health effects, Marcus and his colleagues recruited 100 healthy men and women in San Francisco and equipped them with Fitbits, continuous glucose monitors and electrocardiogram devices that tracked their heart rhythms round the clock for 14 days.

Each participant followed a strict coffee schedule: They were instructed to drink as much caffeinated coffee as they wanted for two days, then to abstain for two days, and to repeat this cycle for two weeks. The participants were told to press a button on their heart monitors every time they drank a cup of coffee to document their intake in real time.

To ensure the participants followed the instructions, the researchers sent them daily reminders and even reimbursed them if they provided date-stamped coffee receipts. They also used a form of virtual monitoring called “geofencing” to track coffee-shop visits.

NEW DATA ENLIGHTENS

On days when they drank coffee, the participants tended to consume about one to three cups, though some drank much less, and a few drank as many as six cups of coffee daily.

Coffee had clear effects on sleep. People got about 7.2 hours of nightly shut-eye on days when they avoided coffee and 6.6 hours on days when they drank it.

Genetics seemed to play a role: People who carry genetic variants that make them what are known as “slow metabolizers” of caffeine had greater reductions in their sleep when they drank coffee compared with “fast metabolizers,” potentially because the caffeine stays in their systems longer. (Many direct-to-consumer DNA testing companies such as 23andMe will tell you if your genes make you a fast or slow metabolizer of caffeine.)

The effects on physical activity were particularly striking. Marcus at UCSF said it’s not clear why people walked an extra 1,000 steps on days when they drank coffee. It’s possible they had more energy and motivation.

Either way, taking an additional 1,000 steps per day is associated with a 6% to 15% reduction in mortality – “effect sizes that are remarkably similar to the magnitude of mortality benefit observed among coffee drinkers,” the study noted.

“That’s a clinically relevant difference in physical activity that may have long-term positive implications,” Marcus said.

COFFEE AND THE HEART

The researchers were particularly interested in how coffee affected the heart. Premature atrial contractions are a type of irregular heartbeat that emanates from the top chambers of the heart, called the atriums, while premature ventricular contractions come from the bottom chambers, called the ventricles. Almost everyone experiences these palpitations on a normal basis, which can feel like your heart fluttered or skipped a beat.

The researchers found that on days when people drank more than one cup of coffee, they experienced about 50% more premature ventricular contractions. While these are not considered dangerous, there’s some evidence that they might be a warning sign in people who experience a lot of them.

One observational study in 2015 that Marcus was a co-author of found that people who routinely experienced many of these palpitations were more likely to develop heart failure. “That doesn’t mean everyone,” Marcus said. “But we do know that the more you have the higher the risk.”

Amit Khera, a cardiologist who was not involved in the study, called it unique and important. He said that most people shouldn’t worry about the potential for coffee to cause palpitations because in healthy adults there’s no indication that they’re dangerous. But he cautioned that the findings did not necessarily apply to people with heart disease.

“In healthy people with normal hearts, it’s what I would call a quality-of-life issue, not a life-threatening issue,” said Khera, the director of the preventive cardiology program at the University of Texas Southwestern Medical Center. “If you feel your heart is skipping and it bothers you, then based on these study results cutting out coffee could reduce those symptoms.”

Peter Kistler, an expert on heart rhythm disorders who studies coffee’s health effects, said that even though people experienced more premature ventricular contractions on days when they drank coffee, the incidence was low and the palpitations “entirely benign.” “People should be reassured that coffee is safe and part of a healthy diet,” said Kistler, the head of clinical electrophysiology research at the Baker Heart and Diabetes Institute in Melbourne.

Marcus said that the finding needs further investigation. But meanwhile, people should tailor their coffee intake to their individual needs. If you need motivation to exercise then consider using coffee to give you a boost. But if you suffer from insomnia or you worry that you’re at high risk of developing heart failure because it runs in your family, then consider cutting back, Marcus said.

“There’s no one size fits all prescription or recommendation,” he added. “It really depends on the individual.”