I know someone who kitted his new-build house out with skylights rather than windows and they allowed a disappointing amount of light in. Some of it would depend on your latitude though.
well, if the sun is directly above the tube, it would get maximum sunlight. the other times of day, it will be at an angle to the pipe, forcing the light to bounce back n forth down the tube, and in turn will be less intense. another problem may be light from the closet going back up the tube. you could just cap it at night i guess