In analysis of milk yield from the milking period, it was shown that the most milk was produced by Group 1 sheep and the least by Group 2 animals. Similar results were obtained by Molik et al. [
2,
5]. This previous research demonstrated that ewes milked in the period of shortening day produced 50% less milk than animals that lambed in the spring and summer. Research by Molik at al. [
3] showed that the introduction of a variable melatonin signal in sheep modulates content of SFA (saturated fatty acid) decreased and the content of PUFA (polyunsaturated fatty acid) increased. Implantation of exogenous melatonin and exposition of ewes to artificial short days (16D:8L) caused a decrease in MUFA (monounsaturated fatty acid) and PUFA content. The study showed that the introduction of biochemical and light modulation of photoperiod through increased melatonin secretion via subcutaneous implants and an artificial short-day condition can affect milk composition in seasonally breeding ewes. Melatonin not only decrease lactose concentration but also caused a significant deterioration in the fatty acid profile of milk. These results imply seasonal variations in milk composition common to Polish milking ewes, which cannot be overridden by farming though diet or breeding system. Further studies are needed to confirm that phenomenon in order to make sheep milk production system more regular around the year together with better manufacturing properties of milk [
3]. Also, the research results being presented here showed that introduction of exogenous melatonin implants in Group 3 sheep 6 weeks before lambing led to reactiveness to melatonin (melatonin resistance), and lactation in this Group lasted 11 days longer than in ewes of Group 2. Throughout the study, it was found that the dry matter content in the milk of Group 1 was lower than in the milk obtained from ewes that lambed in June (Group 2) and those treated with exogenous melatonin (Group 3). Sheep milked in short photoperiod conditions (Group 2 and Group 3) from the beginning of lactation produced milk with a higher dry matter content than sheep of Group 1. In all examined groups of sheep, the content of dry matter was increasing over the course of the lactation period, which had also been confirmed by previous studies [
5]. Protein content analysis determined that the milk of mothers in Group 2 and Group 3 presented a higher protein content than the milk from Group 1 animals. In addition, the milk of sheep from Groups 2 and 3 already in the first month of milking contained more protein than the milk of ewes in Group 1 [
20,
21,
22].
Analysis of changes in fat content in sheep’s milk showed that the milk obtained from Group 2 and Group 3 animals was characterized by a higher fat content compared to the milk collected from animals in Group 1. According to the study by Pavić et al. [
22] fat content increases over the course of lactation. The results collected showed that lactose content in the milk of Group 2 sheep was significantly lower compared to the content of this component in the milk from animals of Group 1. Over the course of lactation, lactose levels in milk systematically decreased in all sheep groups. Similar results regarding changes in lactose concentration in sheep’s milk were obtained by Bonczar and Reguła [
23] as well as Park et al. [
24]. Chemical composition of the milk from ewes in Groups 2 and 3 in the first months of milking was similar to that of the milk from Group 1 sheep over the last two months of milking. Different lambing time (season) and administration of external melatonin did not cause significant changes in calcium content. Only in the third batch of samples, the milk from ewes with melatonin implants (Group 3) was characterized by a higher calcium content in relation to sheep that lambed in February. However, the overall level of calcium in the milk of all the treatment groups was higher than that found in the studies by other authors [
9]. Introduction of melatonin implants had a significant effect on phosphorus content. The milk of sheep milked during short day (Groups 2 and 3) was characterized by a significantly higher content of this component. Content of phosphorus in the milk of sheep that lambed in February (Group 1) was comparable to that determined by Haenlein [
9]. In the fatty acids content assays, it was determined that the milk of Group 1 ewes had a significantly higher content of C4:0, C6:0, C17:0 and C18:0 acids. In contrast, the milk from sheep in Groups 2 and 3 was richer in C8:0, C 10:0, C12:0, C14:0, and C16:0 fatty acids compared to that of Group 1 sheep. In general, the highest content of SFAs was recorded in the milk of Group 3 animals. The SFA levels recorded in the milk of Group 1 sheep were similar to results obtained in the studies by Carta et al. [
25]. Based on the results from MUFA acid assays, the highest concentration of these acids was present in the milk of Group 1 ewes. The milk of animals in Groups 2 and 3 contained significantly less C18:1 (
n = 9), C18:1 (
n = 7) and C18:1 (cis-12) acids. The obtained results align with the research carried out by Meluchova et al. [
7], Carta et al. [
25], and Molik et al. [
3]. The highest content of PUFAs in the experiment was recorded in the milk of sheep in Group 1. The lowest levels of CLA were determined for the milk of treatment Groups 2 and 3. Taking into account the health-promoting and nutritional properties of polyunsaturated fatty acids (PUFAs), it can be assumed based on the research results presented that the milk obtained from ewes in Groups 2 and 3 offers weaker pro-health parameters. The values for linoleic acid recoded in the study are comparable to the results obtained in the studies of Luna et al. [
26], Clare et al. [
27] Meluchova et al. [
7]. Sheep milk offers exceptional health benefits; it owes its properties to the rich chemical composition. This high health value of sheep’s milk results from the presence of inter alia antioxidant substances such as CLA. Conjugated linoleic acid (CLA) is one of the most important antioxidants of milk fat and exhibits anti-carcinogenic and anti-mutagenic effects [
28]. Recently, we observe an increase in popularity of the so-called functional foods, i.e., foodstuffs demonstrating health-promoting properties, resulting mainly from the presence of bioactive substances that have a positive effect on metabolic processes and correct balance of various components [
28]. Currently, also the interest in the possibility of using sheep’s milk in treatment of metabolic disorders in humans is growing [
29,
30]