Because the Fitbit element has been widely covered, it is felt as clear for discussion. For anyone not aware of interface details, I’ve attached an image of heart rate and sleep tracker detail from an Alta HR. If one is awakened from sleep, it is readily evident; start of sleep time is automatically detected if/when it occurs. Reading heart rate is not simple. Spikes can occur from stress and during exertion.
The increase shown before noon was due to an unexpected elevated-cortisol event. It is a different pattern than that which occurs while exercising.
This is not being shown to assist in sleuthing, rather, to illustrate just some of the data the investigation has. Note these are only two features, additional ones exist.