I believe this is correct, but "deep, uninterrupted sleep" does not necessarily occur over longer periods of time. I think the point is that it's easier to get a deep, uninterrupted sleep while the sun is down -- especially if you go to bed at the same time every night.
That last point may be more important than most people realize. I once lost quite a bit of weight during a "change-of-lifestyle" period in my life, and one of the keys was that I was taking a review course for a professional licensing exam on Saturday mornings. This meant that I couldn't stay up late on Friday nights like I was used to doing, and I think this played a big role in my success.