Men who sleep well at night may be at less risk of developing advanced prostate cancer, according to researchers from Harvard School of Public Health and the University of Iceland in Reykjavik. In a study led by Ms. Sarah Markt, a doctoral candidate in the department of epidemiology, men who slept longer and with fewer interruptions had increased levels of the sleep hormone melatonin in their urine and were 75 percent less likely to develop advanced prostate cancer than those with less melatonin.