What does darkness refer to? Light in regards to player-character [visibility][1].  
Why am I asking this? I want optimal conditions for sneaking, so I want to know what time of day I should be performing covert ops.

If you have a suggestion on how to make this question less ambiguous, please comment.


  [1]: http://gaming.stackexchange.com/questions/106918/when-should-i-enchant-with-muffle-instead-of-sneak/106948#106948