Reposting on behalf of Norb because I think this forum might be more appropriate for it.
running the Spotify client prevents my Mac from going to sleep, even when no song is being played.
By typing "pmset -g assertions" into the terminal window, you'll get a list of diagnostic information regarding power save mode. As long as Spotify is running, it contains the the line
PreventUserIdleSystemSleep named: "com.apple.audio.'AppleHDAEngineOutput:1B,0,1,2:0'.noidlesleep"
This would be the expected behavior when a song is being played but seems to be a bug otherwise.
A fix would be nice!
This problem has been around for a while now, I can't believe it hasn't been fixed yet.
^ That is from almost 2 years ago!