How to keep my iPhone 5 from going to sleep when a video is being played?

My iPhone 5 is running the latest version of iOS 7.
Lately, I've been noticing that when I'm following a workout video, it goes to sleep after a while.
This has never happened to me before, and I've had this phone for more than a year.
It's very annoying when your phone just goes to sleep when you're trying to workout.
I use apps to workout, but it's not just one particular app that this has been occurring in.
Can someone please tell me how to fix this please?

Find a setting for it or keep tapping it <3

Try restarting it & see if that helps.

Go to both video settings and display settings.