How does your brain know when to wake up, and when to go to sleep? The secret is circadian rhythm, coming from the Latin, circadian, meaning "about a day."
The brain has an internal clock that affects a lot of bodily functions, such as temperature, when we get hungry, hormone levels, and so forth. Scientists believe this clock also governs when we go to sleep, and when we wake up.
We usually go to sleep at about the same time every night, and for most of us about eight hours later we wake up.
It's believed that we fall asleep at about the same time each night because chemical activity in the brain induces sleep. Then, for most of us, about eight hours later our brain clock switches on chemical activity that wakes us up.