How much sleep do you need? Dr. Suresh Kotagal, a sleep specialist at Mayo Clinic, says that kids up to age 13 need 9 to 9 ½ hours of good sleep. By the time they’re teenagers, that number goes down to 7 or 8 hours. What happens when they don’t get enough sleep? The level of stress hormones and steroids in the body go up. This can cause weight gain. Dr. Kotagal recommends waking up around the same time every day. Avoid napping during the day. And exercise – just not in the two or three hours before bedtime.