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.