Hello and welcome.
I have seen this issue before, in many iterations of Mac OS X. Some things that can cause this:
-Connected USB devices that prevent sleep - especially speakers and device dongles
-When using an Ethernet connection for networking and leaving Wake for Ethernet access checked in the Energy Saver preferences
-Some third-party menulets in the menu bar - i.e. those that monitor system processes like fans, temps, etc. and cloud syncing services like Dropbox
This could be an issue with El Capitan itself, as honestone stated, so it will be interesting to see if it is corrected with a future maintenance release of OS X.
C