iPhone 6 battery went from 100% to dead overnight

I Had charged my iPhone 6 before I went to bed, it was 100% and I randomly woke up 6 hours later to find my phone off!! the battery had completely gone.

I Had before I'd gone to sleep had the software update reminder Come up to which I clicked the remind me later option. It didn't update but I have no idea why it died. I use my phone as my alarm for work so I would have been stuffed if I hadn't woken up. Having to plug my phone in tonight so I can have piece of mind.

Has anyone else had this problem?