Melatonin 

 
Source: Pineal gland

Introduction

It is a major hormone of reproduction and involved in seasonal reproduction. More melatonin is produced during darkness. It is more in winter because the nights are long.

Production of melatonin

 The cells of the pineal gland produce this hormone. The precursor of melatonin is tryptophan. Tryptophan is absorbed into the pineal glands from the blood and converted into serotonin (neurotransmitter). After this, it is converted into N-acetyl serotonin and then it is converted into Melatonin.

How melatonin affects reproduction

It is species dependent. It stimulates reproduction in short-day breeders and inhibits reproduction in ling day breeders. e.g., Sheep is a short day breeder, so more melatonin production occurs during winter and they reproduce in winter while the mare is a long day breeder.