ok, first off, you should be giving your bearded dragon some live food, they need the protein to best grow. I am not sure what type of bearded dragon food you are giving him but look into some bugs for him.
They do need the uvb to digest correctly, to absorb the cacium correctly (by producing their vitamin D). Running around your house, even with the beardie being by the windows, will not let him absorb the correct thing needed for the vitamin D. Glass blocks the UVB rays, and thus will cause the beardie to get nothing from the sun except heat. Not having proper feeding, and not having proper uvb can cause issues with bone, growth, feeding habbits, and shedding. I would suggest getting a
uvb light (reptisun 10.0 tube if you live in the usa.) untill then, if you live in a warm area, take your bearded dragon out for 30 minutes of direct sun. Lots of wonderful information available here on this site, I hope you get your answers you are needing and are able to help your little guy.
As for natural light, I think it is suggested, if you cannot get them a uvb, to atleast give them 30 minutes every other day out side in direct sun, they cannot be under glass, and you don't want to put them outside in a glass cage as that can trap heat, and cause overheating. Some screen's will also block UVB from the sun so make sure there is really nothing between the beardie and the sun. if you use a small swimming pool, etc, give them a hide spot so that he can still regulate his heat. Also you will never want to leave him by himself outside, as you don't know what is walking by, or flying by that might want a bearded snack.