Yes, Seasonal Affective Disorder is real

No matter how cutesy the acronim SAD is. Joseph reports on a study that links SAD to serotonin. But serotonin itself may not be necessary to understand how SAD works, though an intimate link between serotonin and melatonin (the former is the biochemical precursor of the latter) suggests that serotonin should be looked at in this context.
Also, if you suffer from SAD you should be very careful preparing for your long-distance travel: getting jet-lagged may trigger a bout of a few days of depression regardless of the time of year.