seasonal affective disorder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A mood disorder occurring during seasons when sunlight is limited, characterized by symptoms of depression such as dysphoria and loss of energy.

from the GNU version of the Collaborative International Dictionary of English.

• noun A form of mild depression that occurs in winters, associated with reduction in the amount of sunlight. It is characterized by oversleeping, irritability, and sometimes overeating. It can be treated by light therapy and usually disappears with the arrival of spring.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun pathology A form of depression associated with the lack of natural light during the winter months.