They’re spotted on the arms of celebrities from Beyonce and Britney to Victoria Pendleton and Ben Shepherd and nearly 20 million people in the UK bought a fitness tracker in the last year.
The gadgets monitor our heart rate, the number of steps we take and calories we burn, how many flights of stairs we go up and down and how well we sleep.
Paired with apps on our smartphones, we compare our activity levels with friends, set challenges and share our goals on social media.
But while trackers are helping to get sedentary Brits moving, we can become obsessed with relying on their data.
I spoke to five experts about how monitoring our every move could be harming our health for this feature for The Sun newspaper.
Obsessing about our progress makes us depressed.
Sports psychologist Lindsay Woodford of TheSportingMindset.com believes they’re taking the fun out of keeping fit and normalising obsessive and neurotic behaviour.
She said: “It’s the opposite of mindfulness where we enjoy the moment and instead are more concerned on feedback from a watch and fixating on data.
“It’s taking the focus away from the natural endorphins that exercise brings.”
Lindsay has worked with runners who have become ill or injured and felt depressed at being unable to hit their goals.
She explained: “People post data on social media and I’ve had clients who have felt very down when they can’t be part of that community.
“They feel lonely because they can’t run anymore and demotivated so psychologically it can have a big impact.”
Striving to reach our goals leads to injuries.
Personal trainer Geraldine Jackson fears regularly tracking exercise can increase the chance of getting injured.
She said: “I’m concerned about people pushing themselves too hard, not listening to their body and chasing their goals despite being injured.
“Exercise is about reducing stress but trackers can act in a negative way, increasing stress if the user doesn’t reach their target.”
Geraldine thinks it is helpful to track progress, but users should remember devices aren’t 100% accurate.
She said: “Be aware of how active you are. Make time for yoga, a spin class or a swim.
“It’s useful to have this data, but unless you are using it to make changes physically, it could have a detrimental effect and be demoralising.”
Tracking sleep stops us nodding off.
She explained: “People used to ask me ‘how do I know if I’ve had a good night’s sleep?’ and I’d ask how they felt. If they said, ‘great’ they’d had a good sleep.
“But now you might wake up, look at your tracker, see that you slept for seven hours and only four were good and think, ‘How could I possibly feel ok?’
“Sleep is meant to be a state of restfulness and unwinding. Wearing a tracker means you have a heightened sense of awareness and are almost overanalysing what’s happening.”
Sammy says users should focus on how they feel.
She added: “Look at how you perform at work. Are you irritable? Are you moody or irrational as all these things impact on sleep?
“Sometimes we have to change the variables. Cut out caffiene or take off the device for a week and see it makes a difference.”
Calorie counting makes us eat more.
Seeing how many calories we’ve burned off each day can make us reach for the wrong foods.
Rather than having three healthy meals a day, nutritionist and author of The Drop Zone Diet Jeannette Jackson has seen clients consume their daily allowance in one meal.
She said: “People rely more on the stats and figures and less on how they are feeling, tuning out from their natural instincts and using the app to dictate their food intake.
“I’ve had clients who manipulate their calories, so they consume in bulk because they are starving and then eat nothing for the rest of the day.
“Or vice versa where they save most of their calories for an evening out.
“It can be an unhealthy pattern to get into and although apps can have a positive role in helping to develop healthy eating patterns, an over reliance or addiction to them can cause more damage than good.”
Wearing mucky devices leads to skin complaints and viruses.
Users have shared their experience of picking up rashes, blisters and skin irritations from wearables on fitness forums.
According to Dr Peter Barratt, Technical Director at Initial Washroom Hygiene, they can become hotbeds of germs if not washed regularly.
He explained: “Up to 80% of all infections are transmitted by hands.
“Devices like these will come into contact with numerous objects and people creating a high risk of cross-contamination, spreading germs from one surface to another.
“Fitness trackers worn during exercise will naturally pick up moisture and associated germs. Many bacteria and viruses will proliferate at 37°C, our body temperature.
“These germs will mainly be skin-related bacteria, but in some cases, and where hygiene is not as good, could include germs which are be more likely to cause illness too.”
Wearers should clean devices after exercise with antibacterial wipes regularly and wash their hands for between 20 and 30 seconds.