Method to detect a sleep phase of a user wearing a personal activity monitor, said personal activity monitor comprising a motion sensor, a processor unit and a temperature sensor, wherein a set of states are defined regarding the personal activity monitor together with the user, the set of states comprising at least NOT WORN, AWAKE, POTENTIAL SLEEPING, SLEEPING, the method comprising the steps of: Sa- determine via the motion sensor a current sensed level of activity, and compare this current level of activity to a predefined low level of activity, and to a second predefined level of activity, Sb- determine via the temperature sensor a current temperature (STemp) at the vicinity of the activity monitor, and a first temperature profile, Sc- if the sensed level of activity is below the predefined low level of activity for at least a first period of time and if concurrently the first temperature profile meets a first predetermined temperature criterion over the first period of time, then infer that the personal activity monitor is actually worn by the user and that the user has entered the POTENTIAL SLEEPING state, - if the current level of activity remains below the second predefined level of activity and if concurrently the first temperature profile keeps meeting the first predetermined temperature criterion over a second period of time, determine that the personal activity monitor is being worn by the user and that the user has entered a SLEEPING state, and then include a duration of the POTENTIAL SLEEPING state into effective sleep, and otherwise exclude the duration of the POTENTIAL SLEEPING state from effective sleep, Sd- if the sensed level of activity and/or sensed temperature meets a wake activity criterion, infer that the user has emerged from sleep, Se- calculate the sleep duration of the user, between instants of entering and emerging from sleep. Sa- determine via the motion sensor a current sensed level of activity, and compare this current level of activity to a predefined low level of activity, and to a second predefined level of activity, Sb- determine via the temperature sensor a current temperature (STemp) at the vicinity of the activity monitor, and a first temperature profile, Sc- if the sensed level of activity is below the predefined low level of activity for at least a first period of time and if concurrently the first temperature profile meets a first predetermined temperature criterion over the first period of time, then infer that the personal activity monitor is actually worn by the user and that the user has entered the POTENTIAL SLEEPING state, - if the current level of activity remains below the second predefined level of activity and if concurrently the first temperature profile keeps meeting the first predetermined temperature criterion over a second period of time, determine that the personal activity monitor is being worn by the user and that the user has entered a SLEEPING state, and then include a duration of the POTENTIAL SLEEPING state into effective sleep, and otherwise exclude the duration of the POTENTIAL SLEEPING state from effective sleep, - if the current level of activity remains below the second predefined level of activity and if concurrently the first temperature profile keeps meeting the first predetermined temperature criterion over a second period of time, determine that the personal activity monitor is being worn by the user and that the user has entered a SLEEPING state, and then include a duration of the POTENTIAL SLEEPING state into effective sleep, and otherwise exclude the duration of the POTENTIAL SLEEPING state from effective sleep, Sd- if the sensed level of activity and/or sensed temperature meets a wake activity criterion, infer that the user has emerged from sleep, Se- calculate the sleep duration of the user, between instants of entering and emerging from sleep.