When you wake up your bedding will be warm. Making it right away will trap the warm air inside of your bed. Warm air carries more moisture than cold air so when the air cools down that moisture is going to settle in your bedding, making it a perfect breeding ground for all sorts of things you’d rather not have in your duvet, sheets, and pillows. Mites, bedbugs and the likes also like the dark so airing out your bed makes it more uncomfortable for things you don’t want to live there to make it their home.