The definition of seasonal affective disorder (SAD) is a history of major depressive episodes and remissions that are seasonal, affecting mostly women during the winter months in colder countries. Symptoms commonly begin in the autumn, peak in midwinter and subside in the spring. The exact cause is unknown; however, there are several contributing factors including seasonal change of light affecting internal circadian rhythms and secretion of the hormone melatonin, genetic inheritance and diet. Mental health care clinicians are in an ideal position to promptly recognise the condition and implement treatment that may help limit the severity of the depressive symptoms. The degree of illness can vary, as well as the timing of onset and resolution and may depend on geographical location. Treatments include light therapy, diet, exercise, psychotherapy and antidepressant medication.