First time I experienced polyester (rather than cotton) - there's flying sparks of static that can be seen at night like mini lightning strikes all over when you rub against the sheets. Really thin quality, not much material to keep you warm - and the electricity conflicts with the cotton pajamas - only good if you're wearing polyester clothing. With cotton, my skin breathes better - the moisture does not build up under the sheets like it does with polyester. Just sharing, that's all. I love the pattern on the sheets - so cute and funny (colorful)!!