How do they know when to wake up?

Good Morning Think::

My tulips are in full bloom.  I planted over 100 last fall (yea bone meal).  I have so many I don't even feel bad about cutting them to put all over my house. 
How much do you love them in the darling vase you gave me last May?  Where did you get that anyway?

As I was coming home tonight my tulips out side and the ones on my window sill (shown above) all knew it was time to go to bed and gave me no trouble going to sleep, unlike my other three pink tulips (my girls).  How do the tulips just go to sleep like that?  How do they know when to wake up?  I should have taken Botany!

Thanks again for the vase, because I am quite sure I never sent you a thank you note.

Pink::