Heart rate monitoring and sleep tracking

River · January 22, 2026, 7:47pm

The round version does not list a heart rate sensor in its tech specs. How does it do reliable sleep tracking without one? If the sleep tracking software on the round version is unreliable because of the lack of a heart rate sensor, does the rectangular version actually use the heart rate data for sleep tracking, or does it just fall back on the same unreliable software of the round version?

Flynn · January 22, 2026, 10:43pm

Sleep tracking does not currently use the HRM

github.com/pebble-dev/pebble-firmware

src/fw/services/normal/activity/docs/index.md

main


# Health Algorithms
## Step Counting
The step counting algorithm uses input from the accelerometer sensor to detect when the user is walking and how many steps have been taken. The accelerometer measures acceleration in each of 3 axes: x, y, and z. A perfectly still watch resting flat on a table will have 1G (1 “Gravity”) of acceleration in the z direction (due to gravity) and 0G’s in both the x and y axes. If you tilt the watch on its side for example, the z reading will go to 0 and then either x or y will show +/-1G (depending on which of the 4 sides you tilt it to). During watch movement, the x, y, and z readings will vary over time due to the watch’s changing orientation to gravity as well as the acceleration of the watch when it changes direction or speed. The pattern of these variations in the accelerometer readings over time can be used to detect if, and how fast, the user is stepping.

There are generally two dominant signals that show up in the accelerometer readings when a person is walking or running. The first is the signal due to your feet hitting the ground. This signal shows up as a spike in the accelerometer readings each time a foot hits the ground and will be more or less pronounced depending on the cushioning of your shoes, the type of flooring, etc. Another signal that can show up is from the arm swinging motion, and the strength of this will vary depending on the user’s walking style, whether their hand is in their pocket or not, whether they are carrying something, etc. 

Of these two signals, the foot fall one is the most reliable since a user will not always be swinging their arms when walking. The goal of the step tracking algorithm is to isolate and detect this foot fall signal, while not getting confused by other signals (arm swings, random arm movements, etc.). 

An overall outline of the approach taken by the stepping algorithm (glossing over the details for now) is as follows:

1. Separate the accelerometer sensor readings into 5 second epochs.
2. For each 5 second epoch, compute an FFT (Fast Fourier Transform) to get the energy of the signal at different frequencies (called the _spectral density_)
3. Examine the FFT output using a set of heuristics to identify the foot fall signal (if present) and its frequency.
4. The frequency of the foot fall signal (if present) is outputted as the number of steps taken in that epoch.

As an example, if the FFT of a 5 second epoch shows a significant amount of foot fall signal at a frequency of 2Hz, we can assume the person has walked 10 steps (2Hz x 5 seconds) in that epoch. 

### Example Data
The following figure shows an example of the raw accelerometer data of a five-second epoch when a user is walking 10 steps. The x, y, and z axis signals are each shown in a different color. In this plot, there is a fairly evident five-cycle rhythm in the red and green axes, which happens to be the arm swing signal (for every 2 steps taken, only 1 full arm swing cycle occurs). The ten-cycle foot fall signal however is difficult to see in this particular sample because the arm swing is so strong.

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River · January 22, 2026, 10:55pm

From this it doesn’t look like they are even trying to distinguish different stages of sleep. This isn’t sleep tracking at all.

Flynn · January 22, 2026, 10:57pm

Correct. All of the health features are far more limited compared to Apple or WearOS smartwatches. I think I saw someone on Discord a little while back tolking about potentially redoing the health measurements so it may become more sophisticated at some point.

adam · April 30, 2026, 2:48am

On April 25 someone shared a screenshot of the new Pebble Health app on r/pebble that shows the features outlined in Pebble Tips.

Sleep data

Average length
“Deep” sleep average
Fall asleep time average
Wake up time average
Displays data by day, week, and month (I asked on Reddit if you can view all historical data by using the right arrow)

Potential future info

Hopefully the Pebble Time 2 will use the heart rate monitor to improve the accuracy of the current sleep data with future upgrades.

Poysuma · June 5, 2026, 5:45pm

I updated my Time 2 to PebbleOS 4.11.0 today. I woke up at 08:00 and then about an hour later, I took my watch off and then put it back on around 10:00. The watch registered the hour of inactivity from 9:15-10:08 as sleep. That’s not too surprising to me, but strangely it also somehow registered deep sleep from 9:42 - 10:04. So I second the idea that sleep tracking should take heart rate into account. A period of inactivity with no accompanying heart rate should tell the watch that it’s not being worn.

Additionally, since sleep data can sometimes be lost, I think a night of 0 hours and 0 minutes of sleep should not be counted when calculating average sleep time; regardless of the cause, that’s clearly an outlier. My watch crashed when I updated it the other day, causing the sleep data to be lost, and this in turn has introduced bad data into the health tracking. Alternatively, giving users the ability to delete logged sleep as another user suggested would solve this issue as well.

Topic		Replies	Views
Option to delete sleep Product Feedback	2	60	May 31, 2026
Pebble doesn't track multi-phase sleep (PT2) Product Feedback	0	72	May 26, 2026
Questions about Heart Rate Monitor and Step Counts General Discussion	10	579	June 2, 2026
Disable Unneeded Health Features Product Feedback	1	92	May 27, 2026
Pebble “Health” Watch General Discussion	7	402	February 26, 2026

Heart rate monitoring and sleep tracking

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