screen backlight overheat

The phone does that to protect itself, so this is normal if you use the screen that extensively.

If the device exceeds a certain temperature threshold, it will present a temperature warning screen similar to this:

• The device stops charging.
• The display dims or goes black.
• In navigation:

  The device will present this alert and turn off the display: "Temperature: iPhone needs to cool down."

• Navigation will continue to provide audible turn-by-turn directions. When approaching a turn, the display will illuminate to guide you through the turn.
• To return the device to normal operation, press the Home Button and slide to unlock. If the device has cooled down enough, you can continue normal usage.

copied from Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

You're welcome 🙂

If this happens while playing a lot of graphic and processor intensive games, this could be a reason for that.

The processor and the screen are using lots of power, warming up the phone, if this happens during normal use, you should get it checked.

The older iPhone models did not have that kind of "power output".