Our internal clock regulates how tired we get and how lone we sleep, causing the brain to release neurotransmitters which induce sleep. These include the sleep hormone melatonin, which the pineal gland produces in larger quantities during the hours of darkness. The need for sleep is determined by our genes, and varies c considerably from one person to another. Anything between four and 12 hours a day is considered normal, provided a person feels well.
The need for sleep varies between individuals. While a few can happily go for a run before dawn, far more struggle to stay awake if they don't get as much sleep as possible.