What you hear and smell can change your mood. But sleep makes a really big difference. If you don't get enough sleep just one night a week, it can make it hard to think. Being tired can make you stressed, angry, and sad, too. When you sleep enough, you feel good. When you're well-rested, you'll have energy to move, play, or exercise. Until you're 10 years old, you need 10 to 11 hours of sleep a night. Older? Try to get at least nine hours of sleep a night.