Hi, i think i know what is happening. I bought my iphone about 2 months ago, and it was working perfectly, with all the updates. Never did it ever turn itself off whilst in sleep mode, no matter what the battery level. I live in the UK. However, i just went to France for a 2 week holiday. The day i arrived, i went to show my parents my iphone for the first time, and guess what, it wouldnt turn on. I was like, this has never happened before! It proceeded to be unresponsive in sleep mode throughout my stay. I thought i would need to get a new iphone once back in the UK. I have now been back for one week, and the problem has gone completely, and i've been running the battery literally dead the whole time.
I think the issue is a network issue. In the UK, my iphone is set permanently to the O2 network. In France, in the area i was in, the signal was pretty weak mostly, and as such, it would keep changing between three different carriers, Orange, FSFR and Bougues. I can only think that the iphone gets confused or something when it has 3 network signals to choose from, especially when they are weak.
In conclusion, it cant be my phone because its fixed now, it has to be a location based issue, and network issue. I hope this helps.