Maybe got one of those lights that slowly dim to simulate sunset. You can fall asleep in the light, but it will be dark while you sleep. I have 2 Philips branded ones that have worked well for 11 years now.
If you are from a cold area people needed to keep a camp fire alive throughout the night or they died, I'd bet those would have a hard time sleeping without lights on since their ancestors who did died.
DNA adapts very quickly to such hard selectors, and many populations has lived in such areas for many thousands of years by now.
