The human body constantly generates a variety of signals that can be measured from outside the body with wearable devices^[1]. These bio-signals – ranging from heart rate to sleep state and blood oxygen levels – can indicate whether someone is having mood swings^[2] or can be used to diagnose a variety of body^[3] or brain disorders^[4].

It can be relatively cheap to gather a lot of bio-signal data. Researchers can organize...