why do people get sleepy?

why do people get sleepy? and how is it possible to sleep like 10 hours? are you really sleeping 10 hours of your life away, or are you refueling for the good hours? these are the just a few of the questions i wish someone could answer for me. it seems like on the weekends i can go to sleep at 11 o'clock at night and at 2:00 in the afternoon the following day im still not awake! my mom tells me im sleeping my life away, i just tell her im spending time doing something that i love to SLEEP! haha