Why is it so difficult to fall asleep when you are overtired? | Popular Science

To fall asleep, you need adequate time to unwind, even if you feel tired. Most people don't allow themselves enough time to relax before bed.

Lack of sleep makes it worse. When you get less sleep than the recommended 7 to 8 hours a night, you begin to accrue a sleep debt. Your body experiences a stress response and begins to release adrenaline, which stimulates your body to be alert. To overcome this, you need a good night's sleep on a regular basis.