It's to do with the circadian rhythm. It's a cycle our bodies go through which can be daily or many times a day, which control the biological processes.

The sleep itself is controlled by neurotransmitters (melatonin) released from the SCN within the hypothalamus which act on neurons in the brainstem and spinal cord. Depending on light and temperature (which is why people often feel sleepy in winter and merrier in the summer) it makes the body prepare for sleep, such as a lower urine production and the melatonin itself makes you feel drowsy.

It is believed that humans have a 25-hour sleep cycle, therefore it resets itself daily. Blindness can affect the sleep cycle, as can jet lag.

As the rhythm progresses, light will disrupt it again causing us to wake up. This happens by the pulse rate returning to normal as well as temperature rise.

You can wake up just by someone opening a door. This brings the air pressure down in the room you are sleeping, only by a fraction, but just enough to wake you.

There's plenty of evidence that daylight is percieved by the eyes, even with your eyelids closed and that this can trigger wakefulness.