The are many threads on this, with a variety of causes and proposed solutions. From my personal experience, I would ask you to go into menu> settings> about phone> battery and report your up v awake time. If they are the same or near the same (a heavy user would probably have around a 4:1 ratio over an extended period of time or 2:1 over short periods) the phone is not sleeping properly.
I would power off/on, or do a battery pull and restart, leave the phone be for about 20 minutes and see what the times are now at. I do know that sometimes, depending on what the phone is doing internally it may not sleep right away. You may need to leave it for several hours or overnight to really determine if it is sleeping or not.