seasonal affective disorder; SAD
A psychiatric term for depression that occurs at a specific season, typically winter. The abbreviation SAD is also used in Japanese contexts.
See also: 季節性情動障害
冬になると気分が落ち込むのは、季節性感情障害の可能性がある。
If you feel depressed in winter, it could be seasonal affective disorder.
Light therapy is sometimes used to treat seasonal affective disorder.