A method for detecting a subject's stress level associated with an activity includes (a) connecting the subject to a sensor that senses a value of a biometric; (b) during the activity, (i) repeatedly sensing the value of the biometric over each of a plurality of time windows; and (ii) computing, for each time window, a deviation in the sensed values of the biometric; and (c) detecting the stress level based on the computed deviations. In one implementation, the value of the biometric is a skin temperature measurement. The method may be implemented as an application in a wireless handheld device, such as a cellular telephone.