I can sleep in any condition tbh (and by "sleep", I mean nap - as long as my arm is over my eyes). For a legitimate "sleep" I need to be in darkness I guess. I dunno really, I know if the blinds are down, I'll wake up really early due to the light so I have to have them down, but as long as the lights are off, I'm fine sleeping. Even if there's light from another room I can still sleep. Kinda lucky tbh because when I was younger I used to find it hard to sleep now I find it really easy. :3
Oh and since people mentioned it, I go to sleep with a radio on. I find it relaxing. I used to wake up with the radio on but now I don't bother.