Crack in cement basement floor

About 5 yrs ago cracks started forming in my basement floor. Water has never come up through them and the basement is quite dry. (I think the floor may be over a thick plastic sheet since the land has nearly zero percolation. House was built before I bought it 28 yrs ago.) After a year or 2 no new ones started but every winter the old ones get more prominent and then appear to close up slightly in the summer. They do no obvious harm but I'm concerned about them if/when I try to sell the house. Does anyone have a suggestion as to the cause and/or what to do about them, if anything?