http://www.techradar.com/news/the-date-december-2-is-crashing-a-lot-of-iphones-and-ipads-but-apple-has-a-fix

If you've woken up on Saturday, December 2 to find your iPhone or iPad randomly crashing, you're not alone, as posts on Twitter and Reddit confirm. The problem centers on apps that use daily or repeat reminder settings, which apparently cause CPU use to spike before forcing a reset.

Not ideal if you were planning to use your phone for something like making calls or idly browsing the internet today, but the good news is that Apple has already rolled out a fix - install iOS 11.2 on your device and the issue should magically disappear.