Actually chemises aren't even all that body-flattering. Most of them are usually embroidered (if you're rich) cotton, linen or wool...resembling mumus. They have absolutely no shape to them and were worn over pantaloons as sort of a modern day bra and underwear, if I'm correct.
Usually at night women would wear them to sleep in, if you could afford ladies in waiting to undress you from the layers and layers of clothes you would wear in the morning.