you're correct; if you go to bed with the sun shining on you, not only will you have uncomfortable sleep, you will also have an extremely weak immune system and will be more prone to sickness (due to the lack of melatonin). further points are discussed in the bullet lists.

the best environment to sleep in would be a dim/dark room. personally, I'm sleeping in the living room, so I make sure that the blinds are properly blocking potential sunlight to ensure uninterrupted sleep.