Well Rested

Well Rested is a status effect that grants the player character a +10% experience boost for 12 hours after sleeping in an owned bed in Fallout 3, Fallout: New Vegas, and Fallout 4.

Fallout 3
The Well Rested-effect is obtained by sleeping in an owned bed (in the Megaton house or Tenpenny Tower suite, a rented bed, or a cot in Kenny's Cave).

The perk Deep Sleep, available with the Broken Steel add-on, allows geting this status by sleeping in any bed, but only for 8 hours.

Fallout: New Vegas
Like Fallout 3, the effect is obtained by sleeping in an owned bed (e.g. in the Lucky 38 presidential suite or Dino Dee-lite motel room), safe house beds, sleeping with certain characters like Sarah Weintraub, Red Lucy, or Joana at Gomorrah, or specific queen beds in certain locations (such as Jacobstown bungalow).

Fallout 4
The Well Rested effect is now considered a temporary perk and is obtained by sleeping in any unowned bed.

If sleeping while in the company of a companion with a romantic relationship, instead of the Well Rested perk, the Sole Survivor will instead receive the Lover's Embrace temporary perk, which increases XP earned by 15%.