Almost everyone knows newborns need to sleep virtually all day and all night. And, babies need a lot of sleep day and night, too. But, what about toddlers and preschoolers? How much sleep does a 3-year-old need? Your 3 year old needs an average of 11 hours 15 minutes of sleep in a 24-hour period. Most of that sleep, if not all, will be made up of nighttime sleep. 3-year-olds need an average of 9 hours, 45 minutes to 11 hours, 15 minutes at night and 0 to 1 1/2 hours of sleep during the day.