That is definitely a phone issue, however Seidio makes a extended battery and case. You pay a bit more but they're excellent quality
Sounds like you have a rogue app or maybe even a hardware issue. You may want to look more into those before investing in an extended battery. I mean if it's eating up to 40% during the time you sleep then you'll just end up saying the extended battery isn't lasting as long as it should be, and it won't as long as this odd issue remains.
Try downloading a battery monitor app such as GSam. It'll give you a breakdown of what apps are eating up your battery the most and so on. If it's not a rogue app then it could very well be a hardware issue that should be brought up to your service provider or Samsung.