We're diurnal, and have eyes optimised to see maximum colour and detail instead of well in dim light (at least by mammal standards). It makes sense we'd gravitate to fairly dark conditions to sleep, because while nature at night is not perfectly unlit, it's still pretty dark. Darker than a developed-world urban area will ever get, for example.
That being said, many people are completely capable of sleeping in a bright area, myself included.
As for the bonus question, yes, the hormones at least work backwards in nocturnal animals. Melatonin wakes something like a shrew up.