Activis
A description of physiological and behavioural data streams Activis captures from the wearable device: what each stream is, how often it is recorded, and the resolution at which it is stored.
The device is a 24/7 optical wrist tracker. Activis uses the Polar Loop (Gen 2), an optical wrist device that measures heart rate and related signals from blood flow at the skin. Heart-rate variability is derived from optical pulse-to-pulse intervals (the time between consecutive heartbeats).
Almost all data is pulled from the device's own continuous recording, not streamed live. The Polar Loop records continuously on the wrist, on battery, with no phone present. The Activis app connects over Bluetooth periodically and pulls the recorded data off the device. Because of this, the recording frequency of each stream is set by the device's firmware — Activis stores whatever cadence the device recorded; it does not resample it.
All timestamps are stored in UTC at millisecond resolution. Local time is used only for display.
Nothing is filtered or discarded at capture. Values are stored exactly as the device reported them, and the device only clears its own copy once the server has confirmed it holds the data.
The device is the Polar Loop (Gen 2) — an optical 24/7 wrist tracker.
| Stream | What it is | Recording frequency | Stored resolution |
|---|---|---|---|
| Pulse-to-pulse interval | Time between consecutive heartbeats — the basis for HRV, and for beat-by-beat heart rate | One value per heartbeat | Whole milliseconds (the highest-precision stream) |
| Heart rate (24/7) | Continuous heart rate | Samples 1 second apart — 1 Hz during activity, one 5-minute average at rest (device-decided). Beat-by-beat heart rate is also available from the pulse-to-pulse stream above. | Whole beats per minute |
| Activity | Steps, calories and activity intensity | Recorded in device blocks through the day | Whole steps and calories; activity intensity (MET) to two decimals |
| Skin temperature | Wrist skin temperature | Recorded at a device-set interval | °C to one decimal place |
| Skin contact | Whether the device is on-skin (wear time / compliance) | Recorded as on-skin ↔ off-skin changes | On-skin / off-skin state |
| Sleep / wake | Sleep and wake state, overnight and naps | Recorded as sleep ↔ wake changes | Sleep / wake state |
| Sleep phases | The night broken into phases | Per-phase intervals across the night | Wake / Light / REM / Deep, with start and end times |
| Nightly recharge | Overnight recovery summary | One record per night | Autonomic ("ANS") charge, HRV, beat-to-beat, breathing-rate and heart-rate nightly averages |
| Battery level | Device battery (device-health) | Recorded on change | Whole percent (0–100) |
This is the raw signal for heart-rate variability: one value per heartbeat, each the interval to the previous beat, stored to the millisecond. Heart rate follows directly from it (heart rate = 60,000 ÷ the interval in milliseconds). It is derived optically from the pulse, not from an ECG. Each beat also carries a heart-rate value and two quality indicators — an error estimate and a skin-contact / movement flag — so lower-quality beats can be identified.
Heart rate is available beat-by-beat from the pulse-to-pulse stream above — roughly one value per second at rest, and more often when the heart rate is higher. In addition, the device keeps a continuous 24/7 heart-rate file: samples are one second apart, recorded at 1 Hz during activity and as a single five-minute average at rest. The beat-by-beat stream gives the finest resolution; the 24/7 file gives gap-free coverage for periods when beat detection isn't possible (for example during movement, or when the device is off the skin).
PRV is computed on the platform from the pulse-to-pulse intervals of each measurement session; it is not a recorded device stream. Because the intervals come from the optical sensor, the measure is pulse-rate variability (PRV), reported as RMSSD. Intervals outside 300 to 2000 ms are removed. An interval that differs by more than 20% from the median of the preceding five accepted intervals is rejected as an artifact, and any successive difference involving it is excluded. A value is produced only when the accepted intervals total at least 60 seconds and no more than 5% of all intervals in the session were removed or rejected. The removed and rejected counts are stored with each value. The raw intervals are always kept in full.
The device records activity as a step count, calories, and an activity-intensity value (MET, the metabolic equivalent of task) in blocks through the day, at an interval set by its firmware. A once-per-day summary is also recorded: total steps, total and active calories, and active time.
Wrist skin temperature, recorded continuously at an interval set by the device, to one decimal place in °C.
The device records each change between being on-skin and off-skin. This is the basis for wear-time and compliance — it shows when the device was actually on the wrist, and therefore when the other streams reflect the participant rather than an idle device.
The device records sleep and wake as state changes, and stages the night into Wake, Light, REM and Deep phases with start and end times. For each night it also records total sleep duration, the light/deep/REM durations, a sleep-continuity measure, and a sleep skin-temperature reading. Separately, a nightly recovery summary ("recharge") gives autonomic ("ANS") charge, HRV, beat-to-beat, breathing-rate and heart-rate averages for the night. The sleep staging and these summaries are computed by the device.
Recorded whenever the battery level or charging state changes. This is device-health information for monitoring wear and sync, not a physiological measure.