I heard, more progress towards spiritual life or penance will result in more light coming out from body (like God's body, not as much light as God has but still some) means, more we do penance, more our body will start shining with light.
I heard of an incident when Raama did visit Sage Agastya's hermitage during Vanwaasa, then light came from his body and that light was spread in a very large area that the light can be visible even from outside of hermitage.
So is this true that penance makes body source of light? If yes then what kind of light is it? Defiantly, not like Sun's shine because light of Sun contains heat. But body light don't contains heat.