It's normal, just keep an eye out if it happens after whelping. Mother rats can 'zone out' or fall asleep with their eyes open (i think i made my mother do that) because the little ones only want to eat. Keep an eye on this, i lost two rats years ago that way. Best to seperate the babies and the mother at night once they start exploring.
But, for normal pregnancy, it's fine! More than fine, if it doesn't happen i get worried. First time it happened i thought she was taking lessons from the daddy! Lazy... lol