I put my iMac t sleep for the night only to find it on again anywhere from1 min to 1 hour later. Using terminal, I entered "syslog |grep -i "Wake reason", but it did not show anything for the last 48 hours, and in that period, nearly all entries were from XHC1. Just as a test, I just put it to sleep and in about 10 seconds, it woke up. Running the terminal script, again did not show any entries for this time period. Run El Capitan, but the same issue was there for Yosemite.