This is interesting.
As I mentioned in another thread - I saw two Octopuses today at an Aquarium, both of whom seemed to be in a state of deep rest when I first saw them.
Both were a very pale 'yellow' at first, not entirely in synchronous camouflage with their surroundings. One 'woke up' (For wont of a better phrase!) but the other remained perfectly still, apart from breathing. Later, as we passed the tank on our way to the exit - Both had gone into hiding.
From what I observed, I could reach no absolute conclusion as to whether or not they 'sleep' - In the human sense. But certainly, I saw an octopus in a state of deep rest, with little apparent self-concern (Otherwise the camouflage would have been more effective I think)
When humans sleep, they are unaware of their surroundings, and are therefore disinhibited. For creatures (Octopuses) that are normally quite defensive, this resting behavior, and the lack of inhibition it included was certainly indicative of a form of sleep recognisible to humans.