Changing of seasons,amount of sunlight that the spring and summer increases have an affect. Winter, we'd rather hibranate like many animals do, but not offered that luxury because we are human.
The brain's chemistry,and the part of it that governs our sleep/wake cycle according to this change, different hormones involved with this process,are also influenced with the season's various hours of daylight.
Google, circadian rhythm, there may be things that best explain this, if it interests you.
The change in meds can have a temporary effect too,it's best to mention this to the prescribing physician, to rule out anything else that may be causing this, he or she may want to rethink your med combo or dosage changes, best to work with your doc.
Hope all works out for you, take care now.

DE