Aspects of the disclosure are directed to detecting physiological and/or other characteristics of subjects. As may be implemented in accordance with one or more embodiments, a time series of cardiac intervals is computed from a recording of activity of a beating heart of a subject, and the time series is decomposed into subcomponents. An envelope of at least one of the subcomponents is computed, and the presence and/or degree of a depressive or stressed mental state of the subject is detected based upon characteristics of the envelope.