季节性情绪紊乱（seasonal affective disorder，SAD）指多发于冬季的一种抑郁症，也称“冬季抑郁症”。与其它类型的抑郁症不同的是，这种季节性抑郁症会在春天到来时有所好转。这是一种循环的、反复发生的情绪紊乱，每年冬天时觉得抑郁，春天到来时又好转了。

    A study by Alfred Lewy at the Oregon Health and Science University found that most of the symptoms of seasonal affective disorder are caused by body rhythms that are not in sync with the sun. Often the body gets out of sync with the sun because of winter's shortened days. The pineal gland, which regulates body rhythms, secretes a hormone called melatonin during the night. This can affect your sleep wake cycle and other rhythms. When it gets dark too early in the day, the pineal gland and the body's sleep wake cycle get out of sync.

    俄勒冈健康与科学大学的阿尔弗雷德•洛威所做的一项研究发现，季节性情绪紊乱的大部分症状是由身体节律与太阳不同步导致的。通常来说，身体会因为冬季白天变短而无法与太阳同步。控制身体节律的松果体会在夜间分泌一种叫做褪黑素的激素。这种激素会影响你的作息周期和其它节律。天黑得太早时，松果体和身体的作息周期就无法同步。

    Seasonal affective disorder is far more common in northern climates, where days can be very short in winter. SAD affects more women than men and is likelier to occur in people under age 40 than those older than that.

    季节性情绪紊乱在北方气候地区更常见，北方冬季的白天都很短。该症状的女性患者多于男性，而且更易发生在40岁以下人群中。