The altered phase relationship between the timing of sleep and the circadian rhythm of body core temperature has been reported previously in DSPD patients studied in entrained conditions. That such an alteration has also been observed in temporal isolation (free-running clock) supports the notion that the etiology of DSPD goes beyond simply a reduced capacity to achieve and maintain the appropriate phase relationship between sleep timing and the 24-hour day. Rather, the disorder may also reflect a fundamental inability of the endogenous circadian timing system to maintain normal "internal" phase relationships among physiological systems, and to properly adjust those internal relationships within the confines of the 24-hour day. In normal subjects, the phase relationship between sleep and temperature changes in temporal isolation relative to that observed under entrained conditions: in isolation, tmin tends to occur toward the beginning of sleep, whereas under entrained conditions, tmin occurs toward the end of the sleep period—a change in phase angle of several hours; DSPD patients may have a reduced capacity to achieve such a change in phase angle in response to entrainment.