In a famous experiment over 25 years ago, the phenomenon of being unaware of your surroundings when you sleep was demonstrated in a very dramatic way.

Subjects were to lay down on a gurney, or bed. A 50,000 candlewatt strobe light was placed in front of their face. Their eyelids were taped open, and they were wired so that their brainwaves could be recorded.

The subject was given a small button to press every time the light flashed. Also, each time the light flashed, the gurney was jiggled.

The light was repeatedly flashed, at random intervals, and for a long time the subject would accurately press the button with each flash. However, eventually, they would miss pressing the button. The experimenter would then ask the subject "Why did you not press the button?"  And the subject would say, "I didn't see the light flash."  When their brainwaves were examined at that point, they showed the signs of being asleep!  A transition to sleep had occurred. 

Other resources:

Sleep research centers, sleep doctors, sleep clinics