Do Not Disturb schedule problem

I'm having a problem with my iPod touch (I've known this problem on iPhone too) that my Do Not Disturb schedule dosen't work how I want it. I have it set for 22:00 to 7:00 but it seems to stay on after 7:00. If I manually turn Do Not Disturb off it will say 'Scheduled 22:00' and I won't activate at that time.