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There's a nervous excitement. I can see the cold breath emanating from my fellow runners as they perform the "starting line" wiggle on the starting line.

A swift flick of the hips.

I'm participating in my local 5km community parkrun and I'm focused. It's almost time...

Wait a second. I have to start my watch. So does the guy next to me.

The person behind us tuts. I'm not moving until… there! Signal acquired, I'm off.

Amidst the crowd, it's difficult to find a person without a smartwatch on it. And this isn't only us runners who wear them.

It's a explosive, multi-billion pound sector. Companies like Garmin dominate, featuring an broad array of various devices to suit a spectrum of lifestyles. Based on how high tech the model you go for, prices can start from an affordable sum to thousands of pounds.

"I find it infuriating, I can't switch off."

"It comforts me… It's like a companion encouraging me."

"Stop telling me I've experienced a poor sleep."

Just some of the comments I've had from fellow tracker enthusiasts when I inquire what they think of theirs.

A Sea of Statistics

My feelings on my own device? At this moment, frustrating. It's reporting my speed is wrong, leading me to doubt whether the overhead branches along my route are interfering with my location tracking.

Millions of us attach these little monitors onto our arms and wear them 24/7, seemingly at ease with the fact they're continuously tracking us.

A distant memory are the days when we depended on them exclusively to give us an rough count of our daily steps. Now, they can analyse our nightly rest, blood pressure, oxygen saturation, our body battery.

It's tricky to find a health indicator they aren't designed to measure.

But do they do what they say they do? And are the numbers we analyse a faithful picture of our actual health? Do they improve our daily wellness, or do the avalanche of metrics and constant notifications just increase our sense of overwhelm?

A Complicated Bond

"I do get a a little fixated with mine," a wearer admits.

While she values how her device records her runs, she finds some of its other capabilities "excessive" at times - a feeling that became apparent during a major life event.

Prior to discovering she could put it into a specific mode, her smartwatch kept telling her she was being insufficiently energetic enough. Now, the watch frequently reports her she's had a poor sleep score.

"I am caring for a six-month old, I don't need to be notified my sleep was poor," she says. "That fact is only too well."

Why doesn't she just stop wearing it?

"I could, perhaps, but I've got this push-pull dynamic with it," she says. "I appreciate it for its exercise data, I sometimes question whether the myriad other functions it can now do is perhaps overkill for me."

How Do They Work?

Every device of smartwatch has its own proprietary way of tracking your health metrics and processing your data, but the most use optical sensors on the reverse of the watch.

They commonly emit small, green LED lights onto your arm which can observe blood flow, detecting your heartbeat. More advanced devices detect changes in the bioimpedance that runs through the skin to assess your stress levels.

Expert Perspectives

One expert says that in general for smartwatches, it's a "difficult line" - while the continuously improving tech could potentially prevent illness, "spotting abnormalities before we notice anything", it could also turn us device users into the "anxious healthy".

He points out that newer wearables can conduct tests like heart rhythm analysis, which constantly monitor how stable the heart is. They can notify if someone is experiencing atrial fibrillation (AF), where there is erratic electrical activity in the heart causing an irregular heartbeat.

This does not necessarily indicate someone is on the verge of a cardiac event, but it can give an advance notice they might be more susceptible of a blood clot in the years to come.

But how to make sense of these numbers is complicated. The expert fears that as more functions are added, people may not be able to fully contextualise their data.

"I'm not entirely convinced that the ability to monitor so many metrics is such a good idea," he says.

A clinical psychologist specialising in cardiology agrees. Her work into the impact of wearable tech on a specific group of heart condition patients suggested a minority of those who were given devices to monitor their heart health experienced greater stress and were "far more likely to use doctor's time".

She observed a vicious circle with her patients - they saw a alarming metric on their watch. They became worried. Their heart rate went up. They got increasingly anxious. They checked again, and their reading rose further.

"Upon viewing numbers about ourselves that we can't properly interpret then, naturally, we are going to seek clarification," the psychologist says. "We look, we obsess - it becomes a vicious cycle."