The secretion of some hormones increases during sleep (e.g., growth hormone, prolactin, and luteinizing hormone), while the secretion of other hormones is inhibited (e.g., thyroid stimulating hormone and cortisol). Some hormones are tied directly to a particularly sleep stage. Growth hormone is typically secreted in the first few hours after the onset of sleep and generally is released during slow-wave sleep (SWS). Cortisol is tied to the circadian rhythm, and peaks in late afternoon, regardless of the person’s sleep status. Melatonin is released in the dark and is suppressed by light. Thyroid hormone secretion occurs in the late evening.