Your sleeping period is primarily governed by your circadian rhythm - even if other factors (ie duration of previous sleep) may influence the duration of sleep.
The body’s circadian rhythms control the sleep-wake cycle. They play a role in sleep due to how the body and brain respond to darkness, which is when most humans feel tired and tend to sleep.

As darkness sets in, the body’s biological clock instructs the cells to slow down. When the evening becomes dark, the hormone melatonin starts to rise and allows sleep to occur. Melatonin peaks around 2–4 A.M. and then reduces by morning, allowing wakefulness.

There is also a direct relationship between our body temperature and sleep cycle. At the time of lowest body temperature, we find it hardest to stay awake. We will start to feel sleepy at a time when the temperature is falling and be at our most wide awake when the temperature is rising. Body temperature variations throughout the day follow a regular cycle. The highest temperature occurs around 1700 hours and the lowest at about 0500 hours, at which time we are least efficient and the desire for sleep is at its peak.