As a health professional, I have always found that patients with silky pyjamas or nighties always move in bed better. The slipperyness of the material assists patients in transferring in and out of bed. Flannel and cotton materials tend to make you sweat and stick to the sheets, restricting movement.
Always remember to put in a good dressing gown for when you sit out of bed, and a pair of slippers. Remember, your feet may swell a little, so if slippers are tight before you go into hospital, they may not fit later on. You also may not be able to reach down to put them on if you are having surgery so slip on ones are best, but not too loose or they may trip you up.
I believe that as soon as practically possible, you should try and get dressed after an operation. This helps to promote a wellness attitude and assist in recovery. Remember though to keep it loose and comfortable such as a tracksuit or shorts and a tee shirt as you will probably find it difficult to put regular clothes.