Serum melatonin concentration time profiles for different study conditions on night 8. Serum melatonin levels were measured at the end of the study period for 9 h from 20:50 to 06:00. The graph depicts the nocturnal serum melatonin concentration. Points represent mean ± SD. Open circles, healthy subjects on simulated ICU noise and light (NL); solid circles, healthy subjects on NL plus placebo (NLP); solid triangles, healthy subjects on NL plus use of earplugs and eye masks (NLEE); solid squares, healthy subjects on NL plus melatonin (NLM): *P <0.05 at 22:00, 23:00, 0:00, 01:00, 02:00 and 03:00, respectively, for comparison between the NLM and NL nights.