Power being eaten by screen usage
Keep in mind that on the Battery Usage screen even though per-app breakdown always adds up to 100%, it does not represent the full capacity of your battery, but rather only the used portion.  In other words, if your phone is reporting current battery level of 65%, the breakdown is for the other 35%.

Since nothing really happens overnight, it's not unusual for the "Screen" to be shown as taking 90% (again, that's 90% of the used up 35%).  That just means you've used the screen for a little bit, and then nothing else consumed too much battery.

It's possible that you had too little charge left over from the day before and not having the phone plugged in overnight pushed it over the edge.