Why teens stay up late?

It's because their brains naturally work on later schedules and aren't ready for bed. During adolescence, the body's circadian rhythm (an internal biological clock) is reset, telling a teen to fall asleep later at night and wake up later in the morning.

Explanation:

Teens brains work at a later schedule. Due to this they usually stay up late.

Explanation: During adolescence, the body's circadian rhythm (an internal biological clock) is reset, telling a teen to fall asleep later at night and wake up later in the morning.

Adverbial Clause :)

This is correct, and verified by many people.

D. Adverbial Clause