Generally speaking, you rest when you are beat and need to recover your body. However, if you sleep more than 8 hours each day, it may not prevent you from becoming tired in the early evenings. Sleeping anywhere from 7 to 8 hours should be adequate for adults (i.e. 21 years old or older) and 11 to 12 hours for people who are younger than 21 years old unless they are infants.