The present disclosure describes a method and a system implementing the method for determining an optimal time window for sleep of a person. The method comprises receiving samples of a distal skin temperature of the person, and detecting a temperature change pattern in the samples of distal skin temperature. The temperature change pattern indicates a reference point within the circadian rhythm. The optimal time window for sleep is determined on the basis of the indicated reference point. The temperature change pattern is in the form of a drop in the distal skin temperature followed an increasein the distal skin temperature, where the drop and the increase occur within a time window of ten minutes or less.