I Am Nearsighted But When I Wake Up In The Morning My Vision Is Crystal Clear?

I have noticed that sometimes when I wake up I can see nearly perfectly fine. Normally I cannot even tell there are words on some posters in my room, but some days as soon as I wake up for about an hour I can read them!
Normally I wear glasses for driving , but do not need them all the time . I have near sightedness in both of my eyes, and astigmatism in my left .
Is this normalll??