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 .