Your sleep time is not measured from the time you press Sleep (Start Tracking Sleep) to the time you press End Sleep Session, but rather refers to the time that the app actually tracked (estimated) you sleeping.
During each sleep session, tracking begins the first time you enter deep sleep (also known as “Slumbering”) and ends the last time you enter shallow sleep (going from “Slumbering” to “Dozing”).
Depending on the environment in which you track your sleep, your sleep may not be properly recorded.