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) or a Queen Sized bed that can be found in the ruins of buildings, like Alexandria Arms or by having the perk added in the broken steel DLC.

The effect lasts for twelve hours from the time you awake, regardless of how long you sleep. If you sleep in any bed it will cancel the effect but if it is a Queen Sized bed or an owned bed it will return immediatly.

(Sleeping in a Queen Sized bed does not seem to work any differently than sleeping in a regular sized bed -- neither will give you the Well Rested effect if you do not own nor rent them, but neither will cancel the effect either. A handy place to test this is the town of Minefield. Go into the first house (Gibson's) and upstairs are twin sized and Queen size beds. Assuming you have already slept in your own bed, try sleeping an hour in each sized bed and you will see that you do not lose Well Rested. Next Wait for 12 hours to lose Well Rested and try sleeping an hour in each. You will not get the Well Rested effect from either sized bed. What may confuse some people is that if you have, say, 5 hours left on the Well Rested effect and sleep for 6 hours in a bed which you do not own nor rent, when done you will get the message that you are no longer well rested -- not because the bed canceled the effect, but because the original 12 hours has expired.)