Pain scores in the neck, shoulder, arm, leg, and low back according to frequency of night-shift naps during the previous month . The nap frequency was quantified within a 0% to 100% range (0% = no nap at all, 100% = nap taken during every night shift). Musculoskeletal pain at the current time was rated on a four-point scale (0 = almost no pain, 1 = mild, 2 = moderate, 3 = severe). Error bars represent standard deviations. *P < 0.05.