A team of researchers that thoroughly investigated the Apple Watchs ECG sensor claimed that the data from the device may be used to create a reliable and effective stress prediction tool.

Researchers from the University of Waterloo in Canada, utilizing the ECG sensor in the Apple Watch Series 6, reportedly found a strong correlation between individuals reported stress levels when the readings were obtained and ECG data, particularly cardiac acceleration and deceleration capacity.

According to the research, a prediction model was developed utilizing these data and machine learning techniques. It is claimed that the stress models offer a "high level of precision" but a poorer recall.

The study concluded that the Apple Watch has "promising" potential for stress prediction and hypothesizes that even more data points might be incorporated into stress models to increase predicted accuracy because the device captures additional health data, such as sleep and activity data.

A Mayo Clinic study recently disclosed that as cardiac dysfunction is asymptomatic and people are unaware of it , hence it often goes undiagnosed. According to the article, the researchers also believe that the Apple Watch might be used to support mental health treatment by offering exercises like breathing exercises to counteract stress signals and respond quickly to changes in mental health. In the meantime, a recent study found that the Apple Watch can aid in the detection of silent cardiac illness.

Hence, these results demonstrated that "consumer-watch ECGs recorded in nonclinical situations can detect patients with heart malfunction" and be helpful in early treatment. 

(Source: https://health.economictimes.indiatimes.com/news/health-it/apple-watch-ecg-sensor-can-predict-stress-level-accurately-study/96620121)