Why do they call it Morning Sickness?

I have it all day, starts in the morning, but it's 3:50 and I just got sick. I have been throwing up all day since lunch. I had Mexican, and now I think it is on my list of things not to eat. I really hope that the sickness subsides in a few weeks and does not last the entire time. My poor sister-in-law was sick the entire 9 months with my niece and nephew.