spring fever

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

• noun A feeling of restlessness, excitement, or laziness, brought on by the coming of spring.

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

• noun idiomatic A feeling of invigoration and restlessness associated with the arrival of the warm weather and renewal of nature in the spring season.
• noun idiomatic A feeling of laziness or listlessness associated with the arrival of the warm, comfortable weather of the spring season.