Well Rested

A status effect that grants the player a +10% experience boost for 12 hours after sleeping.

The  effect is obtained by sleeping in a bed you own (i.e. in your Megaton house, your Tenpenny Suite, or a rented bed). The effect lasts for twelve hours from the time you awake, regardless of how long you sleep. If the player sleeps in any other bed it will cancel the Well Rested effect.