“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 your statistics (unlike the Intense Training perk), it does not give an advantage when leveling up - there's no change to the number of skill points gained and the increased stats don't 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 your Perception and/or Intelligence is already higher than 8) to Medicine, Repair, Science, Energy Weapons, Explosives and Lockpick.
fo4 Unlike in previous games, you are allowed to have SPECIAL stats higher than 10, so even if your Intelligence or Perception is higher than 8 (or 7 for rank 2), you 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): you gain increased VATS accuracy (from higher Perception) and have greater experience gain (from higher Intelligence).
fo4 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.
fo4 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 your Pip-Boy, then into your skill tree, then back back out (sometimes just going into your 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 your Pip-Boy, backing out, and standing up (to test).