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A sense of building tension. Apparent is the frosty air emanating from my fellow runners as they do the "starting line" shuffle on the start line.

A rapid movement of the hips.

I'm lined up for my local 5km community parkrun and I mean business. The countdown begins...

Oh, hang on. I need to adjust my device. The same goes for the guy alongside me.

The person in the line sighs. I refuse to go until… there! GPS ready, and I'm running.

Amidst the crowd, it's hard to spot a arm without a smartwatch on it. And it's not just us participants who use them.

We're witnessing a explosive, multi-billion pound sector. Brands like Apple Watch lead the market, all offering an broad selection of different devices to suit a spectrum of lifestyles. Depending on how advanced the device you select, price tags can range from an affordable sum to thousands of pounds.

"It drives me insane, I can't stop checking."

"It reassures me… It acts as a supporter guiding me."

"I wish it wouldn't say about a restless night."

Just some of the reactions I've had from fellow device users when I inquire what they their opinion is on theirs.

The Monitoring Conundrum

My feelings on my own device? Currently, irritating. It's indicating my speed is wrong, making me wonder whether the overhead branches I'm running below are blocking my GPS connection.

Millions of us attach these small gadgets onto our wrists and wear them constantly, ostensibly at ease with the fact they're perpetually recording us.

Long past are the days when we used them exclusively to give us an idea of our pedometer reading. Today, they can measure our sleep patterns, heart rate, glucose levels, our recovery score.

It's tricky to find a bodily function they aren't designed to measure.

But are they able to do what they say they do? And are the numbers we analyse a faithful picture of the truth? Do they improve our overall health, or do the multitude of metrics and motivational messages just contribute to our daily stresses?

A Love-Hate Relationship

"I do get a bit fixated with mine," a wearer confesses.

While she loves how her device tracks her runs, she finds some of its other features "daunting" at times - a feeling first discovered during a pregnancy.

Until she found out she could adjust the settings, her smartwatch kept telling her she was not being energetic enough. Later on, the watch frequently reports her she's had a poor sleep score.

"I am caring for a six-month old, there's no point to be notified my sleep was poor," she says. "I'm acutely aware all too clearly."

Couldn't she just remove it?

"It's possible, theoretically, but I've got this complicated dynamic with it," she says. "I appreciate it for its fitness insights, I just wonder whether the additional features it can now do is maybe overkill for me."

How Do They Work?

Every device of smartwatch has its own unique way of tracking your bodily functions and interpreting your data, but the majority use detectors on the back of the watch.

They commonly shine tiny, coloured LED lights onto your wrist which can detect pulsation, identifying your heartbeat. Higher-end devices analyse changes in the bioimpedance that flows through the skin to assess your nervous system activity.

The Professional Viewpoint

A professor says that for most people for smartwatches, it's a "difficult line" - while the ever-evolving tech could realistically aid early diagnosis, "spotting abnormalities before we feel unwell", it could also turn us device users into the "worried well".

He points out that newer wearables can carry out tests like medical-grade monitoring, which constantly monitor how normal the heart is. They can flag if someone is experiencing an irregular rhythm, where there is erratic electrical activity in the heart causing an arrhythmia.

This does not automatically signal someone is about to have a cardiac event, but it can give an advance notice they might be more at risk of a blood clot in the long term.

But how to interpret these figures is complex. The expert worries that as further capabilities are incorporated, people may find it hard to fully understand their data.

"I'm not entirely convinced that being able to monitor so many things is such a positive development," he says.

A professor focusing on wearable tech agrees. A study into the impact of wearable tech on a set of individuals with health issues suggested a minority of those who were given devices to monitor their heart health experienced heightened worry and were "far more likely to use healthcare resources".

She observed a cycle with her patients - they noticed a concerning reading on their watch. They felt anxious. Their pulse went up. They got more worried. They looked a second time, and their reading rose further.

"If we see data about ourselves that we lack context for then, naturally, we are going to want to know more," the psychologist says. "We monitor, we re-check - it becomes a vicious cycle."