Latest BIOS was applied to my XPS 13 9350 last night using the Dell Updater software, I think its version 1.6? Anyway the battery drained overnight because it did not go to sleep. Checked the event logs and as usual nothing to indicate what's waking it up, and no wake timers showing . Tried to send it to sleep again a number of times and it would wake immediately. Sleep worked fine on previous BIOS 1.4.4, Anyway I wonder if Dell treat the "fail to sleep" symptoms as highly as other hardware issues when releasing BIOS patches. For me its the number one issue, my laptop must go asleep correctly and stay asleep otherwise the battery will be useless from charging it too frequently. I had to downgrade the BIOS to the previous version, that seems to have cured it. I'll wait to see if there's further news on this.