Mac doesn’t go to sleep after inactivity

Before updating to Mavericks, my Mac would go to sleep after 15 minutes of inactivity, which is usually 10 minutes after I turn the screens off using Shift+Ctrl+Eject. However, since the update, the Mac no longer goes into standby. I know this as the status light no longer pulses.

I have checked the energy saver settings, and the Mac is set to sleep after 15 minutes so I'm not sure why it is not going to sleep.

Any ideas? Could it be an application that’s still running in the background?

You can get a list of power assertions by running pmset -g assertions:

$ pmset -g assertions
10/27/13, 8:48:55 AM GMT
Assertion status system-wide:
   BackgroundTask                 1
   PreventDiskIdle                0
   ApplePushServiceTask           0
   UserIsActive                   0
   PreventUserIdleDisplaySleep    0
   InteractivePushServiceTask     0
   PreventSystemSleep             0
   ExternalMedia                  1
   InternalPreventDisplaySleep    1
   PreventUserIdleSystemSleep     1
   NetworkClientActive            0
Listed by owning process:
   pid 189(coreaudiod): [0x0000000100000617] 00:00:19 NoIdleSleepAssertion named: "com.apple.audio.'AppleHDAEngineOutput:1B,0,1,2:0'.noidlesleep"
   pid 9879(backupd): [0x00000001000005f1] 00:13:17 PreventUserIdleSystemSleep named: "Time Machine backup"
   pid 9879(backupd): [0x0000000c000005f0] 00:13:17 BackgroundTask named: "Time Machine backup"
   pid 18(powerd): [0x00000009000005e1] 00:13:47 ExternalMedia named: "com.apple.powermanagement.externalmediamounted"
   pid 18(powerd): [0x00000011000002e1] 00:00:00 InternalPreventDisplaySleep named: "com.apple.powermanagement.delayDisplayOff"
    Timeout will fire in 299 secs Action=TimeoutActionTurnOff
Kernel Assertions: 0x4=USB
   id=500  level=255 0x4=USB mod=10/26/13, 6:38 PM description=XHC1 owner=AppleUSBXHCI
   id=501  level=255 0x4=USB mod=10/26/13, 6:45 PM description=EHC2 owner=AppleUSBEHCI
   id=502  level=255 0x4=USB mod=10/27/13, 8:48 AM description=EHC1 owner=AppleUSBEHCI

In the output above, Time Machine is preventing idle sleep.