YouTube hasn't been a huge problem for me - I micro-sleep watching YouTube. It actually puts me to sleep. Very soothing.
The worst for me has been highly emotionally engaging conversations online. I stayed up until 4:30 AM for one of those. That was the ride of my life though - such deep feeling. I regret nothing.
Also listening to Red while playing Chima Online was bad, too. Heavy metal music + in-game combat will keep me awake and alert. But not nearly as bad as forum conversations. I love typing these long posts. Oy.