Serotonin pulses through our brains, across gaps between our nerve-endings called synapses. In a functioning brain, some of the serotonin remaining in these synapses between pulses will be collected back up by the original nerve-ending, rather than to the target nerve-ending, making it unavailable for use.
After all the fall festivities and the joy and warmth of the holiday season, the shorter days and longer nights of snow and rain can wear on a person. Colder climates are tough for everyone, in body as well as in mind. It all can nonetheless be an isolative feeling, despite its pervasive nature and the severity with which it can manifest itself. Indeed, sometimes the post-holiday slump can be something more. Sometimes, it can be seasonal depression.