Screen is almost always the biggest battery user on the G3. As a rule, 30-40% in my case.
If you phone was effectively on stand by all night, it should have only have dropped a couple of % anyway (the standby usage on the G3 is excellent). What was you battery percentage before the phone slept, and then when you used it again in the morning?
76% is huge, but it's relative. If your phone is on, say, 95%, and then you use it for a few mins before going to bed, the phone will then hardly use anything until you next use the phone, so the 76% could actually be 76% of 2%, with the last thing you've done accounting for the most recent battery usage stats - I hope this is making sense?!