Kirstin Mills made some interesting discoveries about her health when she reviewed her fitness tracker data.

Fitness trackers and sports watches have come a long way in recent years.

I last looked at such devices three years ago, and even since then the technology has moved on.

It seems every second person has a Fitbit or an Apple Watch, and there seems to be something to suit every budget. They usually come with an app or the ability to link into third-party apps so you can upload your data and analyse it.

Even if you don’t exercise much or don’t care to keep track of it, you might be surprised how useful one of the devices can be – nowadays they can be just as much about health data as they are about tracking your exercise habits.

Some devices allow you to keep track of your calorie intake as well as calories burned and your weight. I don’t use this functionality, but I have a friend who does, and she finds it helpful to keep her honest about what she is eating. Your device should provide more accurate data on calories burned than say a treadmill or a general calculation will give you.

Some trackers and watches also measure blood oxygen levels and many also measure sleep quality. The level of sophistication will vary depending on your device. I got a new sports watch for Christmas and if I wear it to bed it breaks down the sleep into deep, light, REM and awake. I’m not sure how accurate it is but if I use the same device it’ll show me trends over time.

Some devices, like mine, also allow you to track physiological stress (not to be confused with emotional stress). My watch provides a stress score based on the changes in the variability of my heart rate. High stress can mean I am overtraining or am getting sick. Since I got it it’s always said I’ve had enough restful moments to counter any stress – even if I’ve done a big workout.

Recently I was laid low with a fever for the better part of a week. When I reviewed the week’s data once I was well again I could see the watch had been picking up I was becoming unwell on the Monday before I even started feeling sick that afternoon.

It also showed me that on the Tuesday, when I took Panadol to mask the symptoms (allowing me to carry on as usual) I was stressing my body by not resting.

On the Wednesday I decided to take no more masking medication and consequently felt dreadful and was forced to rest; the stress score went down.

Of course, I was reviewing all this after the fact, but it was a good lesson in listening to your body and perhaps in future I’ll check out what the watch is telling me earlier – as soon as I start feeling sick.

I did not get the watch for the health data – the only reason I updated the model I already had was because the watch can hold music and podcasts.

This meant that – once I had some Bluetooth headphones – the watch was the only device I needed when going on a run or to the gym. However, my brief illness has made me more interested in other aspects of what it has to offer.

Fitness devices also obviously help monitor workouts and that data can also help you healthwise.

For example, my watch gives every workout a training effect score based on my heart rate and speed, so I know if I’m not working hard enough to get any benefit, maintaining fitness, improving fitness or overreaching.