“When the sun is down, a Night Person gains +2 to both Intelligence and Perception (up to a maximum of 10). This perk directly affects your "internal clock" and remains active both inside and outside.”— Fallout 3 in-game description
With this perk, the player gains +2 to both Intelligence and Perception (up to a maximum of 10) between 6:00 P.M. and 5:59 A.M. in-game time.
As this perk only gives a temporary increase to the player character's statistics (unlike the Intense Training perk), it does not give an advantage when leveling up - there is no change to the number of skill points gained and the increased stats do not count for any perk requirements. Thus, the only effects are enhanced threat detection, Intelligence and Perception ability checks (in dialogue and elsewhere), and a bonus of +4 (or less if the player character's Perception and/or Intelligence is already higher than 8) to Medicine, Repair, Science, Energy Weapons, Explosives and Lockpick.
Unlike in previous games, the player character is allowed to have SPECIAL stats higher than 10, so even if the Sole Survivor's Intelligence or Perception is higher than 8 (or 7 for rank 2), they will still gain benefit. Moreover, the benefits are a bit more direct than in previous games (where it mostly helped subtly in indirectly raising skills or passing speech checks): VATS accuracy is increased (from higher Perception) and there will be a greater experience gain (from higher Intelligence).
The night vision will always be active when sneaking indoors, regardless of the time of day. This is generally helpful but may get too bright in well lit rooms.
The night vision bonus may remain active while standing / during daylight hours (6:00 A.M and 6:00 P.M.). This causes the sides of the screen to be blurred when standing, and can cause things to be WAY too bright during daylight hours/indoors. Fixes (works on Console too):
Fix 1: While standing, go into the Pip-Boy, then into the skill tree, then back back out (sometimes just going into the Pip-Boy and backing out works).
Fix 2: While Standing, go to any container and enter the 'transfer' screen, then back out.
Force Bug to Happen: The bug may be manually activated by crouching, waiting for night vision to activate, then going into the Pip-Boy, backing out, and standing up (to test).