As others have mentioned already, there are logs on your phone, referred to as panic logs, that record the reason your phone is restarting. It sounds like you have a bad or damaged sensor that is causing the phone to execute what's called a kernel panic, which is an attempt by the phone to recover from an unrecoverable error. Every three minutes it scans its sensors and if it finds one it can't read, it logs the error then reboots the phone.
You can read all about it in @flannelist's excellent Wiki article on Kernel Panics here.
[[iPhone Kernel Panics|iPhone Kernel Panics - iFixit|new_window=true]]
You don't necessarily need to study the whole thing and learn everything there is to know about kernel panics, but use the guide to locate and download the latest panic log you can find. Take a screenshot or cut and paste the first page or so into your question and we'll be able to analyze the contents and give you a pretty good idea of what's going on with your phone.
[guide|21499|Adding images to an existing question - iFixit Repair Guide|new_window=true]