According to an aspect, there is provided a method of sending a message to a subject, the method comprising obtaining (30； 502) measurements of one or more physiological characteristics of a subject over time； determining (32； 504) a psychological state of the subject for a plurality of time periods from the measurements； and in the event that it is determined that the subject is in a psychological state of interest in a first time period, sending (34； 514) a message to the subject in a time period subsequent to the first time period in which it is determined that the subject is not in the psychological state of interest.
