Melatonin levels in healthy subjects on the baseline night and on the simulated ICU noise and light (NL) night. Serum melatonin levels were measured in all subjects on baseline and NL nights for 9 h from 20:50 to 06:00. The graph depicts the nocturnal serum melatonin concentration. Points represent mean ± SD. Solid circles, healthy subjects on the baseline night; solid triangles, healthy subjects on the NL night: *P <0.05 at 0:00, 01:00, 02:00, 03:00 and 04:00 for comparison of the baseline and the NL night.