Melatonin regulates the wake-sleep cycle by causing drowsiness and lowering body temperature, but which specific chemical pathways lead to this drowsiness?
This is a quite complicated question, as melatonin has quite a range of effects on the cells. First, melatonin binds to three different melatonin receptors, which are found in a wide variety of tissues or organs including brain and retina, cardiovascular system, liver and gallbladder, intestine, kidney, immune cells, adipocytes, prostate and breast epithelial cells, ovary/granulosa cells, myometrium, and skin.
Two of the receptors a G-protein coupled receptors (GPCR), the MT1 and MT2 receptors. The third is a quinone reductase called QR2.
For more details, see this papers:
The role of melatonin is pretty broad, see the figure (from this article:"Melatonin: Nature's most versatile biological signal?")
The receptors transport the outside chemical signal (the melatonin) to the inside and activate (or repress) a number of different signal transduction pathways. See the figure below (taken from Qiagen):
This includes MAP-Kinase pathways, proteinkinase C, IP3, adenyl cyclase and so on. This is quite complicated and I recommend reading some of the following articles if you are interested in a deeper insight. Finally these signals are transmitted into the nucleus and result in a changed gene regulation/expression.
From wikipedia article on melatonin receptors (with some modifications) :