Peat says this:
Having enjoyed the mild climate of Mexico, I became very conscious of the harm done to us by northern winters, and began developing the idea of "winter sickness." In 1966-67, allergies, PMS, weight gain, colitis, and arthritis came to my attention as winter-related problems, and I assumed that the high-latitude incidence of MS related to what I was seeing and experiencing. Studies in Leningrad began revealing that mitochondria are injured during darkness, and repaired during daylight. I observed that hamsters' thymus glands shrank in the winter and regenerated in the summer; shrinkage of the thymus gland is a classical feature of stress, and usually reflects the dominance of cortisone, though estrogen and testosterone also cause it to shrink. Winter's darkness is stressful in a very fundamental way, and like any stress it tends to suppress thyroid function. In the hypothyroid state, any estrogen which is produced tends to accumulate in the body, because of liver sluggishness.
Peat also says this:
Serotonin and its derivative, melatonin, are both involved in the biology of torpor and hibernation. Serotonin inhibits mitochondrial respiration. Excitoxic death of nerve cells involves both the limitation of energy production, and increased cellular activation. Serotonin has both of these actions.
In hibernating animals, the stress of a declining food supply causes increased serotonin production. In humans and animals that don‚Äôt hibernate, the stress of winter causes very similar changes. Serotonin lowers temperature by decreasing the metabolic rate. Tryptophan and melatonin are also hypothermic. In the winter, more thyroid is needed to maintain a normal rate of metabolism.