This clearly seems to be a hardware problem with the iPhone 5 where some white iPhone 5 users seem to notice a small amount of leakage of light from the power connector region of the phone. You may not even notice the leakage in normal lighting conditions but it gets clearly visible in dark environments.
Solution: Take it to the store and they will replace it
Acknowledging the problem with the device, Apple will willingly replace the phone with one that works without light leakage. Remember to take it to the store while the phone is in warranty.