While it probably isn't harming him, I don't know if it's advised to put put a light on it's side like that. Mostly because in nature, the light comes from the sun which is up above, and ultimately what we're trying to do with our cages is mimic their natural habitat as best as we can, so a light on it's side probably isn't 'normal' to them, which in turn might confuse/annoy (maybe even stress?) them.
that's how I see it but in the end it's your call