The present invention relates to an apparatus (100), system (200), method (300), and computer program for distinguishing between active and inactive time periods of a subject. An input unit (110) receives time-dependent activity data (120) (e.g., corresponding to a level of activity). An activity threshold providing unit (130) provides an activity threshold (140) for the subject. An activity assessment unit (150) classifies the time-dependent activity data (120) based on the activity threshold (140). The activity threshold providing unit (130) provides the activity threshold (140) individually for said subject. The present invention provides an approach to patient-tailor the activity threshold (140) for periods of activity and inactivity.
