Melatonin is produced in the pineal gland, which is outside of the blood-brain barrier, acts as an endocrine hormone since it is released into the blood.
By contrast, melatonin produced by the retina and the gastrointestinal (GI) tract acts as a paracrine hormone.
Melatonin can suppress libido by inhibiting secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland, especially in mammals that have a breeding season when daylight hours are long.
The reproduction of long-day breeders is repressed by melatonin and the reproduction of short-day breeders is stimulated by melatonin. During the night, melatonin regulates leptin, lowering the levels; see leptin.
Light/dark information reaches the suprachiasmatic nuclei (SCN) via retinal photosensitive ganglion cells, intrinsically photosensitive photoreceptor cells, distinct from those involved in image forming (that is, these light-sensitive cells are a third type in the retina, in addition to rods and cones).
These cells represent approximately 2% of the retinal ganglion cells in humans and express the photopigment melanopsin.
The sensitivity of melanopsin is consistent with that of a vitamin A-based photopigment, with a peak sensitivity at 484 nm (blue light).
This photoperiod cue entrains the circadian rhythm, and the resultant production of specific "dark"- and "light"-induced neural and endocrine signals that regulate behavioral and physiological circadian rhythms.
Melatonin is secreted in darkness in both day-active (diurnal) and night-active (nocturnal) animals.
Further Reading
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