Aristotle describes sleep in "On Sleep and Wakefulness". Sleep takes place as a result of overuse of the senses or of digestion, so it is vital to the body. While a person is asleep, the critical activities, which include thinking, sensing, recalling and remembering, do not function as they do during wakefulness. Since a person cannot sense during sleep they cannot have desire, which is the result of sensation. However, the senses are able to work during sleep, albeit differently, unless they are weary.