Is melatonin a direct messenger or a second messenger?

Melatonin is primarily considered a direct messenger, as it is a hormone that is released into the bloodstream and directly affects target cells by binding to specific receptors. It plays a crucial role in regulating circadian rhythms and sleep-wake cycles. While it can influence intracellular signaling pathways, it does not function as a second messenger like cyclic AMP or calcium ions, which relay signals within cells after an initial stimulus.