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, or sleeping with certain characters like Sarah Weintraub, Red Lucy, or Joana at Gomorrah.

There's also a queen bed in the bungalow in the last row of Jacobstown, which gives the Well Rested bonus when slept in.

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%.