Seasonal affective disorder (SAD), also known as winter depression or winter blues, is a mood disorder in which people who have normal mental health throughout most of the year experience depressive symptoms in the winter or, less frequently, in the summer, repeatedly, year after year. The US National Library of Medicine note that "some people experience a serious mood change when the seasons change. They may sleep too much, have little energy, and crave sweets and starchy foods. They may also feel depressed. Though symptoms can be severe, they usually clear up". The condition in the summer is often referred to as Reverse Seasonal Affective Disorder, and can also include heightened anxiety.
One of my lecturer affected by SAD. Currently he living at United Kingdom to pursue his PhD. Hopefully he will be fine soon.