Focused in Spring

When spring starts to blossom, and I know it's a gradual process (since it's snowing in Utah today).  I start to feel more energetic, focused, and overall happy.  I wonder what the biological reasons are.  I know that many Psychologists know for a fact that their customer base for sessions is up in the winter and goes down in the spring and summer.  Is it the Vitamin D?  Are we biologically directly affected by the sun?  I think I'll need to do some research and find out.