Teenagers are not getting enough sleep, and it’s affecting their mental health, according to U.S. News & World Report.

According to the Centers for Disease Control and Prevention, about 41 million Americans get six or fewer hours of sleep per night. For teens, it’s even worse.

For most adolescents, nine hours of sleep is ideal. Unfortunately, very few are actually managing that. In fact, surveys show that less than 9 percent of teens get enough sleep. And the amount of rest they get decreases as they progress through high school. In fact, Cornell University psychologist James B. Maas, PhD, a leading sleep expert, goes so far as to call American teenagers “walking zombies” because they live on so little sleep.