In addition to the heart rate sensor, Rob ter Horst also tested the sleep tracking of the Apple Watch Ultra 2. The reference device was a Hypnodyne ZMax, which records brain waves via EEG and is said to record sleep phases with 72% accuracy, according to studies. For comparison, the polysomnography (PSG) used in sleep labs has an accuracy of around 83%. In comparison to the ZMax, the Apple Watch Ultra 2 showed an accuracy of about 73% for REM and deep sleep and a pretty impressive 86.5% for light sleep.
Compared to the competition, this result is excellent and even slightly surpasses the Oura Gen 4 Smart Ring. The Samsung Galaxy Watch Ultra 2, Whoop 4.0 and Garmin Fenix 7 are notably less accurate in this respect. However, it should be noted that this test and comparison only involved one unit of each wearable and a single person. Due to the small sample size, the results are only moderately conclusive of the overall performance and precision of said devices.