Why does a sad person sleep a lot?

It could be stress, depression, anxiety or trying to avoid problems, sometimes anemia but since you said sad it could be depression cause when im depress I either hear music or sleep.

I sleep a lot when I'm sad(which is all the time) because I want to escape from my feelings and my problems and the only way I know how to escape is to sleep so that's what I do.I can't feel the pain that someone caused me when I'm sleeping.Also I do have severe depression so I spend all of my days in my bed all day.

depressed. They have nothing else to do

because when they are asleep they can dream about anything and they can get away from reality especially if they have a bad life, it's like you're in your own world where you can do anything. also because sleeping is just good :)