These learning sessions are not exercises or self-hypnosis. Training sessions are conducted in a darkened room with the learner in a reclined position and eyes closed. The instructions begin by telling the person to relax and just let go, detaching from thoughts or physical distractions or trying to solve problems. In each session the teacher reviews tensing one particular muscle group. A modification of the technique is "biofeedback" in which one uses external measuring devices to indicate how successful one is in relaxing and then to use those techniques to relax without the help of external measuring devices. Progressive muscle relaxation is suggested to be applied daily. The technique has its own top-down sequence, beginning with the upper body and proceeding to the lower parts. Typically, the duration of a course is eight weeks, with each weekly session each lasting an hour or so. Progressive muscle relaxation technique requires a high degree of personal motivation, as the effects are often seen only after prolonged sessions. Before starting a course it might be recommended to consult with a doctor in order to ensure special care and attention is paid to selected muscle groups, as well as to obtain a recommendation on the ideal course. Jacobson's progressive relaxation has remained popular with modern physical therapists.[5] Although many institutions and individuals offer progressive muscle relaxation training, what they teach is not standardized, and no credentialing process is available for progressive muscle relaxation technique instructors.