Seasonal affective disorder, which occurs from the end of December through March, causes episodes of depression, increased appetite, daytime sleepiness and reduced energy. Researchers have determined rates of depression and sadness among the general population of the Netherlands were highest in summer and fall due to heat. Examples of increased aggression were identified with rising temperatures. A study in the 1980s determined eight weather variables corresponded with mood outcomes. These variables included: hours of sunshine, precipitation, temperature, wind direction, humidity and barometric pressure. Researchers determined high humidity caused reduced concentration. Sleepiness and increased pleasantness occurred in low humidity conditions. Netherlands researchers also identified four categories of reactions to the weather: “Summer Lovers,” “Rain Haters,” “Summer Haters” and “Unaffected.” Fastcast: Chilly.