Natural lighting has few negative effects unless a person has migraines or a seizure disorder. Some allergy sufferers cannot stand light from even a 25-watt bulb. Also, some people become photosensitive if their contact lens have scratched the cornea of the eye. Some people also cannot tolerate halogen-type light sources.
Otherwise, in healthy persons, natural light has no untoward effects.
In the outdoors, a person can get a sunburn whether the day is cloudy or full sun. Doctors now recommend wearing sunblock even in the winter, when sun rays reflect off of ice and snow. Over-exposure to UV light can lead to skin cancers. This is also true if using a tanning bed without skin protection.