We all know that being sick feels dreadful, but have you ever wondered about the actual effect that sicknesses like colds and flu have on your body? Getting a cold or the flu often results in excess mucus, headaches, coughing, sneezing, and more, but there are also measurable changes to your body’s wellbeing metrics that can help unpack just why getting sick takes so much out of you.
I recently spent a whole week with various flu/cold symptoms, and after finally shaking most of the illness, I decided to have a look back at the health data from my Apple Watch Ultra 2, which I wore during the days I was sick.
I don’t use the Apple Watch for sleep tracking, so I don’t have Apple’s actual Vitals data, but even diving into some of the regular metrics, I was shocked at what I found. Across a few various and pretty important metrics, there was a clear and obvious change as I was ill, with things clearly returning to normal as I got better.
It’s a fantastic insight into how devices like the best Apple Watches can help track your health and even alert you to potential sickness ahead of time. Here are the six ways my body changed during my illness, according to my Apple Watch data.
1. Heart Rate
I started to get sick the day before my 30th birthday (February 16), while travelling from Edinburgh to York for a weekend away. The first big change I can see in the data is my heart rate. I didn’t do any intense exercise while I was unwell (nothing more strenuous than walking), but there was a clear uptick in my heart rate from the date I took when I was unwell, and the following days I was battling the sickness.
My heart rate was clocked much higher earlier in the month when I’d recorded workouts, but during the seven or so days I was unwell, there was a clear heart rate elevation.
There’s no cause for concern here, a cursory Google search will show you that it’s very common for your pulse to increase in response to sickness. From the British Heart Foundation: “It’s common for your pulse rate to increase in response to an infection or fever. This happens as your heart pumps harder to deliver oxygen and immune cells around your body, helping to fight the infection.”
From the data, my heart rate increased almost exactly with how rough I was feeling, gradually returning to normal as my sickness retreated.
2. Resting heart rate
Clearly linked and also visible on the data was an increase in my resting heart rate. Resting heart rate is the number of heartbeats per minute at rest. Because I was sick and my body was fighting with an immune response, even when I wasn’t out walking or doing anything, my overall heart rate was elevated. I clocked a resting heart rate of 60bpm on February 17, but by February 20 it was as high as 66bpm.
3. Blood Oxygen
I tend to take smartwatch blood oxygen readings with a pinch of salt, as they can sometimes be quite erratic. However, my data shows a very clear dip in the few days following my sickness that correlates almost exactly with my increased heart rate. Again, a quick Google search will show you this is a common symptom of illness, caused by more difficult and labored breathing, phlegm in the airways, and more.
4. Heart Rate variability
Heart Rate variability is used in readiness tools like Garmin’s Body Battery feature to provide metrics as to how able you are to train. As Garmin notes, HRV is the variation in time between consecutive heartbeats, because your heart doesn’t beat regularly like a metronome. As such, a higher HRV can indicate lower stress and a better state of wellbeing.
If you’re sick, your nervous system responds and your heart rate not only goes up, but the beats become more regular. That’s why my data shows a very noticeable drop in HRV right after I got sick, with a slow recovery over the following days. In one study, HRV was even shown to be a possible predictive marker for how severe COVID-19 infections could be, with a higher HRV drop corresponding to a more severe illness.
5. Walking Heart Rate Average
Walking Heart Rate Average is a bit like resting heart rate for when you’re on the move. It measures your bpm while walking at a steady pace. Keen to make the most of my trip, I battled through my sickness to do some sightseeing, and recorded a corresponding jump in my walking heart rate average as a result, correlating almost exactly with my resting heart rate and overall heart rate jumps.
6. VO2 Max
VO2 Max is a marker of your overall cardio fitness, the maximum amount of oxygen your body can use during exercise. The higher it is, the fitter you are.
As someone who does mostly strength training, my cardio is so-so, but has been on the up since the turn of the New Year. Imagine my surprise when, despite doing no exercise during my sickness, my Apple Health app recorded a significant drop in VO2 Max corresponding exactly with my sickness.
This is likely a culmination of several other changing metrics including all the aforementioned heart rate and blood oxygen data, but I was impressed to see Apple Health recalibrate my VO2 Max during my illness despite the fact I wasn’t actually doing any exercise.
Apple Watch’s cold and flu predictions
My musings here are mostly an exercise in curiosity, but speak to the very precise and changing fitness tracking metrics you can find on today’s best smartwatches. My Apple Watch data and the Apple Health app tracked changes in my health metrics that correlated almost exactly to how I was feeling during my illness. It’s enough to make me reconsider tracking my sleep with an Apple Watch and using the company’s watchOS 11 Vitals app, with an important caveat.
I think this data is really interesting to look back on, but I don’t know how helpful it is ahead of time. If you’re anything like me, hearing from a smartwatch that you’re about to get sick, or that you’ve had a rubbish night’s sleep, can sometimes amplify the feeling of illness or tiredness and become a self-fulfilling prophecy.
One of the reasons I’ve shied away from sleep tracking in recent years is this exact problem. I’d wake up to my sleep score, tracking graph, or some other metric and base my whole outlook for the day based on what my phone or watch told me, rather than how I actually felt. Perhaps it’s time to give it another go and see if I can find a happy medium.
Clearly, however, any smartwatch worth its salt is sensitive to changes in your body that can indicate or even forewarn you of illnesses. That might be convenient or helpful for something like a cold or the flu, but when it comes to more serious conditions like Atrial fibrillation or an elevated heart rate, the outcome could be lifesaving.
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stephen.warwick@futurenet.com (Stephen Warwick)
