Only while the sun is up; and even then, the sky is there, but you actually look at everything else for the vast majority of the time. If the ground is as brig

Only while the sun is up; and even then, the sky is there, but you actually look at everything else for the vast majority of the time. If the ground is as bright as the sky -- for example, if it's covered in snow or water that reflects most of the light hitting it -- then that's quite a bit more dangerous than watching a screen all day (see snow blindness). Just because things are "natural" doesn't mean they're automatically safe, or good for you.