Rested

Sleeping is the process of resting in a bed, sleeping bag/roll, or pile of hay to fully restore your health, magika and stamina.

As long as you are not a Werewolf and do not have the lover stone active, sleeping for any length of time also confers a bonus to the rate at which all skills increase, lasting for eight hours (game time). A message on screen tells you which bonus you have received, for example, "You awaken feeling Well Rested".
 * In a normal bed, you receive the Rested bonus, which is a 5% bonus to skill increases.