Have you ever noticed that when you have mosquito bites, they won't itch all day -- and then suddenly, one will itch, and then they'll all itch at once?
And it's almost always first thing in the morning, or at the end of the day; typically very little itching goes on during the day.
It must be a histamine thing. I bet you that they are time released or something.
Let's Bing it and see what we come up with, shall we?
And I got....nothing.