Abstract: Method of an arrangement for monitoring sleep in a subject by measuring within a prescribed interval skin temperature of a predetermined region of the subject's body and a motion sensor for sensing motion of the subject, comparing the measured skin temperature of the predetermined region with a predetermined temperature threshold, and classifying the subject as being asleep or awake based on whether the skin temperature of the predetermined region is above or below the temperature threshold and on the motion data. In alternative aspects the invention relates to methods of and arrangements for manipulating sleep, as well as monitoring or manipulating alertness.