An interesting news story: http://www.bbc.co.uk/news/magazine-16964783
The business of sleeping in one uninterrupted 6-8 hour session is a modern phenomenon, largely aided and abetted by cheap artificial lighting. There are common literary and historical references to a “the second sleep”, and a period of wakefulness during the night commonly devoted to prayer and reflection.
This is interesting in it's own right, but itt sets my imagination going too. Fictionalising this, it’s like the discovery of an extra day in the week, potentially with supernatural or otherworldly visitors, a gateway into something rather strange. Again, there’s a possibility of using this as a framing device for stories in the graphic novel, with the nocturnal visitors of the Second Sleep meeting and talking with a variety of people.