Background: Sleep deprivation is common in critically ill patients in intensive care units (ICU). It can result in delirium, difficulty weaning, repeated nosocomial infections, prolonged ICU length of stay and increased ICU mortality. Melatonin, a physiological sleep regulator, is well known to benefit sleep quality in certain people, but evidence for the effectiveness in ICU sleep disturbance is limited. Methods/Design: This study has a prospective, randomized, double-blind, controlled, parallel-group design. Eligible patients are randomly assigned to one of the two treatment study groups, labelled the 'melatonin group' or the 'placebo group'. A dose of 3 mg of oral melatonin or placebo is administered at 9: 00 pm on four consecutive days. Earplugs and eye masks are made available to every participant. We plan to enrol 198 patients. The primary outcome is the objective sleep quality measured by the 24-hour polysomnography. The secondary outcomes are the subjective sleep quality assessed by the Richards Campbell Sleep Questionnaire, the anxiety level evaluated by the Visual Analogue Scale-Anxiety, the number of delirium-free days in 8 and 28 days, the number of ventilation-free days in 28 days, the number of antibiotic-free days, ICU length of stay, the overall ICU mortality in 28 days and the incidence and severity of the side effects of melatonin in ICU patients. Additionally, the body stress levels, oxidative stress levels and inflammation levels are obtained via measuring the plasma melatonin, cortisone, norepinephrine, malonaldehyde (MDA), superoxide dismutase(SOD), interleukin-6 (IL-6) and interleukin-8 (IL-8) concentrations. Discussion: The proposed study will be the first randomized controlled study to use the polysomnography, which is the gold standard of assessing sleep quality, to evaluate the effect of melatonin on the sleep quality and circadian rhythms of ICU patients. The results may recommend a new treatment for ICU patients with sleep deprivation that is safe, effective and easily implementable in daily practice.