Sense of Time

Most of us have a sense of time. I never realized that dogs also have one. Shelby likes to go to sleep at 8pm. He sleeps upstairs in my room. Ever since Hurricane Sandy he got used to being there. No matter what time he is walked, at promptly 8pm he barks and runs up to the kitchen gate to be let upstairs. In the morning, he gets up with me at 6:15am. I can go to the bathroom during the night and he never stirs. When I go to the bathroom at 6:15am, he jumps up and waits at the door. My actions are the same during the night as they are at 6am. How does he know the time? Last time I checked, he was not wearing a watch.