The 45% is 45% of how much battery has been used though and not 45% of the total battery capacity. If you used an app or played a game for a while you would see the 45% drop as the percentages of what has used what changes.
It's probably just sorting itself out for one reason or another, not a very scientific answer i know but in my experience if you just put up with it for a few days Android will have worked out what it is doing and the jobs a good'un.
Facebook, WhatsApp and other social apps running in the background can cause Android 'System' to use more power.
- Try put the phone in Do Not Disturb mode during the night?