In 2024, we observe the fortieth anniversary of the publication, where, for the first time, the term of Seasonal Affective Disorder (SAD) was used. Presently, SAD is regarded as a special category of mood disorder. In the American Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-V), the seasonality makes a specifier, “with seasonal pattern”, both for recurrent depression or Major Depressive Disorder (MDD), and for Bipolar Disorder (BD). The most spectacular phenomenon among SAD is winter depression. Its symptomatology is mostly similar to atypical depression, characterized by excessive sleepiness and carbohydrate craving. SAD can pertain up to one fifth of persons with MDD or BD and is more frequent in women. SAD can be considered as an extreme expression of the central nervous system (CNS) changes related to circannual rhythm of lighting. The seasonal changes of the CNS function apply to the secretion of melatonin, the neurotransmitters dopamine and serotonin and the hypothalamic-pituitary-adrenal axis. In the circadian and circannual processes, a significant role is played by so-called “clock genes”. SAD may be a legacy of Homo neanderthalensis. In winter depression, the therapeutic effect is obtained by an exposition to bright light (bright light therapy – BLT) as well as to blue light or using the method of dawn simulation. The therapeutic effect of phototherapy has also been ascertained in non-seasonal depression. As a counterpoint to this mechanism, promising trials have been undertaken in manic states by light restriction using blue light blocking glasses.