Information has mainly been gathered by comparing seasonal variations of behavior with climatic changes. Although pollution, changes in air pressure, rapid changes in temperature, solar activity and humidity all have been discussed, most of the attention has been on the influence of changes in temperature and length of day on human behavior. Since the early 1980s, differences in mood and behavior between summer and winter have been of great interest. The most frequent form of SAD is winter depression, which is associated with increased weight and sleep. The frequency of winter depression increases with increasing latitude, and young women are the most affected.
Most authors link winter depression to changes in length of day.
Spring and Fall Until Rosenthal et al. Researchers speculate that disturbances in the sleep-wake rhythm may cause these seasonal peaks. The rapidly changing length of day around the equinoxes might disturb the circadian and sleep-wake rhythms and change mood and activity in vulnerable individuals. In studies of variations of homicide, the results are more conflicting. D'Mello et al.