Halcyon Loop

It looks and feels right

Credit where it is due: the Loop is the best-looking first-generation smartwatch we have seen. The 38 mm aluminium case is small enough for narrow wrists, the bezel is thin, and at 41 g with the woven band you forget it is there.

The display is excellent - 1000 nits, readable in full sun, with a properly black background that hides the bezel edges. The always-on mode is legible without being distracting.

Halcyon used standard 20 mm quick-release pins rather than a proprietary connector, which means the entire aftermarket band ecosystem works. That is a genuinely user-friendly decision and it is the last one on this list.

The heart rate sensor is the problem

We ran the Loop against a chest strap across fourteen sessions: steady-state runs, interval work, cycling and weights. On steady-state efforts it was within 3 bpm, which is fine.

On intervals it fell apart. Ramping from 130 to 175 bpm, the Loop lagged by 20 to 30 seconds and undershot the peak by 12 to 18 bpm, and on three occasions it locked onto cadence instead of pulse - reporting a rock-steady 168 while the strap showed 149 and falling.

Every optical wrist sensor struggles with intervals. This one struggles more than any current device we have tested, including trackers costing $80. If you train by heart rate zones, this watch will actively mislead you.

A fitness tracker whose heart rate readings you cannot trust is a bracelet with a clock in it.

Sleep tracking that disagrees with itself

Sleep staging from a wrist device is an estimate, and we do not expect laboratory accuracy. We do expect internal consistency. On two of seven nights the Loop's summary screen and the detailed graph in the app reported different total sleep times, off by 40 and 55 minutes respectively.

Total sleep duration was broadly plausible against a reference device. REM and deep sleep splits were not - the Loop consistently reported roughly twice the deep sleep of the reference, which flatters the user and means nothing.

Halcyon's support team acknowledged the summary discrepancy as a known bug when we contacted them, and said a fix was in testing. It was not fixed by the end of our review period.

The subscription

Out of the box the Loop shows you today's numbers. Anything longitudinal - weekly trends, resting heart rate over time, a recovery score, or exporting your own data in any format - requires Halcyon Plus at $6.99 a month.

Charging a subscription for cloud analysis is defensible. Charging a subscription to see a line graph of numbers your own device recorded, and to export data about your own body, is not.

Over a realistic three-year ownership period that is $251 on top of the $249 hardware. The watch effectively costs $500, and at $500 it is competing with devices that measure heart rate correctly.

Verdict

Five point eight. The Loop is a well-made object that fails at its primary job. If the sensor were accurate, the subscription would be a mild irritation on an otherwise good product. It is not accurate, so the subscription is the second-worst thing about it.

If you want a watch that tells the time, shows notifications and looks good, it does all of that competently for six days per charge. That is a $249 watch with an unusually good screen.

If you want it to measure anything, buy something else. We will retest if Halcyon ships a sensor firmware update, and we will say so loudly if it works.