Heat, thunderstorm and half the night awake – but my smartwatch says I slept well

Jan Johannsen

29/6/2026

Translation: Veronica Bielawski

Pictures: Jan Johannsen

Smartwatches deliver fascinating data. But you shouldn’t trust every reading blindly. After a hot summer night spent mostly awake, my smartwatch credited me with over eight hours of restful sleep.

I’ve just spent several weeks testing the Xiaomi Watch S5. In the process, I noticed differences in how it calculates calories burned and active minutes – and, during one warm summer night, glaring discrepancies between my own perception and its readings.

Smartwatches

EUR219,62

Xiaomi Watch S5 46mm

46 mm

5

Lying in bed isn’t the same as sleeping

The heatwave has hit northern Germany: when I get into bed during a weekend getaway around midnight, it’s still 25 degrees outside, and the draught only cools the bedroom slightly. Even so, I fall asleep quickly. But I wake up again not long after. It’s simply too warm. Since there’s nothing I can do about it, I just lie there quietly.

Between 3 a.m. and 4 a.m., a thunderstorm rolls over us, complete with heavy rain and rounds of lightning. I lie in bed and observe. Afterwards I fall asleep again. And wake up again. It’s still too warm. Around 8:30 a.m., I give up and get out of bed: the pool is calling.

One glance at the Xiaomi Watch S5 leaves me stunned. Apparently, I slept for eight hours and 15 minutes. According to the watch, I was awake only once. That must have been the one time I briefly got up during the thunderstorm. It’s true that I was in bed for over eight hours. But I definitely wasn’t asleep for all that time. If the watch had shown me four, five or six hours, I probably wouldn’t have questioned it. But eight hours made me suspicious.

Sensors, readings and algorithms

Smartwatch sensors are very good at measuring body metrics and tracking movement. But there are figures they can only calculate rather than measure directly. These include sleep, calorie consumption and active minutes.

Smartwatches can’t record brain waves. Instead, they calculate when and how deeply you sleep from your heart rate, movement and likely other data, too. The results vary depending on each manufacturer’s choice of algorithm.

In the case of the Xiaomi Watch S5, I get the feeling it credits me with sleep for the entire time I’m in bed. The Huawei Watch Fit 5 Pro did a better job of registering the exact moment I woke up – without being fooled by the fact that I like to stay in bed for a few extra minutes.

Compared to the Huawei, the Xiaomi smartwatch also logs more calories burned and more active minutes. On the other hand, the Huawei gives me a higher VO₂ max reading, and it draws on my phone as an additional data source. As a result, it also credits me with additional steps while cycling.

Use the data as a guide, but don’t compare too much

Even though I’ve been quite critical of the Xiaomi Watch S5 so far, it does actually do plenty right. The basic data looks consistent, battery life is long and GPS tracking is accurate. The only thing I can’t get it to do is connect reliably to my Pixel phone, and I haven’t been able to work out why.

Comparing different watches makes it clear that none of them delivers exclusively reliable data. As such, it’s more sensible to track how your own figures develop over time than to compare them with other people’s. After all, nobody knows exactly how accurate their data is, or how heavily the given algorithm weighs each factor.

Jan Johannsen

Senior Editor

Jan.Johannsen@galaxus.de

As a primary school pupil, I used to sit in a friend's living room with many of my classmates to play the Super NES. Now I get my hands on the latest technology and test it for you. In recent years at Curved, Computer Bild and Netzwelt, now at Digitec and Galaxus. more

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