why does Android OS using so much battery

[AC Question]

I am using a Samsung galaxy s3 NEO. This morning I woke up and my phone had 100% battery. After a little time it was still 100% and really hot. I didn't use any apps but it was so hot that I had to turn it of. After restarting my phone it was 75%. So then I checked my battery in the settings menu and I saw that android OS was using 70% of my battery life. Can someone tell me what is wrong with android OS or how I can stop it. Sorry for my bad English