A mare's reproductive cycle is controlled by her hormones, which in turn are influenced by exposure to sunlight. From early spring to late fall, she is in the estrus phase of her cycle, during which eggs mature and are released every 19 to 22 days.
So-called "marish" behavior---aggressiveness, impatience and general grumpiness---is more common during estrus because of increased hormone levels.
As days become shorter, the mare's body produces the hormone melatonin, which shifts her into anestrus. During this period, no eggs are released. The transition to anestrus begins in late fall. By winter solstice, a mare will be in the deepest part of the phase and may seem more calm and easygoing.
Not all behavior changes in mares are hormone-related, however, and it's wise to investigate them even if you think you know the cause. If your mare's personality seems different this winter, start recording your observations, including checks of her vital signs, in a daily journal. Call your veterinarian if she shows any other signs of illness. Continue the journal through the spring and summer, noting her reaction in various environments and situations, such as being in the pasture with other horses or being tacked up for riding. Comparing her attitudes during the winter and summer can help you identify possible hormone-related behaviors.