Why do i feel nauseous and dizzy after focusing on light in dark room?

I sometimes use my iphone late at night with the lights out. When i am focusing on it in the dark especially when i dont have my glasses on, i always get a severe headache and nausea the morning/day after. It is debilitating and makes me want to throw up. Why does this happen and how should i treat it? please dont say just stop using my phone at night.