I think this is a very beautiful piece of art. First of all, your anatomy is flawless and I love how lively the pose is (even if it's of Altair asleep). There's so much personality in your lineart and I absolutely adore your artistic style. The sepia tones make the overall picture warm with the slightest edge and I just love it!
Most pictures I see of Altair look cold and maybe even a little insensitive. Don't get me wrong, they're still very nice pieces of art, but this one sticks out from the rest. I love his faint grin and open demeanor. As I already mentioned, this picture packs too much personality and I could honestly stare at it for an hour. I know this is supposed to be a critique... but I honestly can't find anything to criticize! Keep up with what you're doing; you're an amazing artist!
Well, with all the side quest before the Assaassination and the need to run all across town to get away from the Templars, I'm sure he would need a quople of days of sleep, not just a night. Beautiful painting, and great shading. I always suck at shading.