Historians tell us that we used to sleep in chunks. What? We would sleep for 4-5 hours, wake up, and then go back to sleep for 4-5 hours. This was regarded as normal. Now, we view this way of sleeping as abnormal. This may not be the case. In fact, you are getting your recommended 7-9 hours but in two chunks. Sleep scientists call this polyphasic sleep.
Check out the Polyphasic Sleep Society for more information.
Sleep Tip #175: Sleep in shifts or chunks.