Spring Fever (syndrome)

Spring fever is any of a number of mood, physical, or behavioral changes, which may be experienced coinciding with the arrival of spring, particularly restlessness, laziness, and even amorousness.

Though spring fever is not a medical term (and thus it is not considered a disease or malfunction of the human body), many researchers have documented noticeable patterns of behavioral changes in those studied during the springtime.

The term spring fever is an auto-antonym (a term with multiple and opposed meanings):

On the one hand, the term may refer to an increase in energy, vitality, and sexual appetite, as well as a feeling of restlessness, associated with the end of winter. This concept may have a biological basis. A lift in mood with the arrival of spring, and longer periods of daylight, is often particularly strong in those suffering from seasonal affective disorder (SAD), who experience lows or depression during the winter months. It is this sense that inspires the use of the term as a title for several works of literature and entertainment.

On the other hand, the term may sometimes be used to describe an opposite effect of springtime lethargy or depression.

...
Wikipedia