They tend to preen up the nest topping it up with hay, straw, fur, shredded newspapers etc... and they cover and uncover the babies presumably so she can clean them/feed them? But I think this usually happens out of human sight about 2am...
We had no idea Madam was pregnant (we had her 2 wks!) so we opened the bedroom and disturbed the nest as we wondered why she hadn't come out for breakfast! So she had a little "tidy" of it after we had gone and made sure her babies were warm, covered in shredded newspaper amongst other things!