Re: My XPS: a bomb in my backpack
I've had this issue with the 9550 too. It appears to be caused by failed automatic switchover from Sleep to Hibernate after a preset time. The switchover fails, and the laptop wakes instead, to this weird state from which it doesn't want to sleep/hibernate/turn off again.
First do a hard shutdown (5 sec pwr button). Then reflash BIOS (same version - this seems to reset something). This just to get it back to normal.
Then the workaround is to set Hibernate After in advanced power profile options to Never (0).
Some said this was related to devices connected (wifi mouse dongles).
Check also with powercfg (admin cmd window) if you have any "wake-armed" devices, and disable. Also set no wake timers on battery (can have them on AC power, for windows updates etc).
Sleep is now reliable for me, as long as I don't forget to charge for too long. Hibernate isn't unfortunately, sometimes cold boots instead.