You get a -1 modifier to both Intelligence and Perception from the hours of 6:01 AM to 6:00 PM. You get a +1 modifier to these statistics from the hours of 6:00 PM to 6:00 AM.
J.E. Sawyer's Fallout RPG
When performing actions at night (6:00 PM to 6:00 AM), you do things better. During the day, however... It has nothing to do with the level of light, just the time of day. You're not a morning person. +2 to all skill checks at night, -2 to all skill checks in daytime.
Notes
It was replaced with the Sex Appeal trait in Fallout 2 in all but the German version of the game (in which it "fills the gap" left by the censored Sex Appeal trait).
