Why does my brother sleep longer than the average kid?

My little brother (12) sleeps way longer than me. It's always around 12 to even 14 hours. Sometimes he wakes up after 10 hours, but then goes back to sleep. He is kind of overweight, but he goes outside for like an hour and a half each day. I usually sleep for 8 - 10 hours. Is there any disease that could cause this or is it normal for some people?