Effects of Deep Sleep

Usually i sleep little late at night around 12.00. But last night i was very tired and slept at 9.30 P.M. Suddenly i woke up in the middle of sleep , because  i saw some light through my room window. I thought, it was morning, and the sun is up. To cross verify that, i checked my mobile, the time was 11.30 P.M.
This is the perfect example of a "Deep Sleep"