Melatonin is a special hormone with many functions. Its production in human and animal organisms is particularly seasonal and related to physiological processes. This study monitors blood plasma melatonin concentration in sheep during the whole gestation and early post-partum period, taking into account the effect of the season that has been less studied so far and in certain details for the first time. It proves that nocturnal plasma melatonin concentration in pregnant ewes increases between the autumnal equinox and the winter solstice in Central Europe in the Northern Hemisphere. It observes that nocturnal plasma melatonin concentration (between 18:00 p.m. and 06:00 a.m.) in pregnant ewes follows a less pronounced variation. Furthermore, it provides proof that nocturnal plasma melatonin concentration in pregnant ewes does not change as pregnancy progresses. It is the first to report that nocturnal plasma melatonin concentration decreases to the same level in ewes and new-born lambs immediately after birth, without nighty fluctuations.