Not to be nitpicky, but I don't believe larger dinosaurs would have been able to sleep lying down. Like horses and elephants, their weight would cause too many issues--they'd basically crush themselves. Chances are they'd lock their knees and sleep standing up. Though you probably did it the way you did because drawing a dinosaur sleeping standing up would be tough. It'd just look like it's standing there. Not as creepy for it to be snuck up on by a carnivore.
Eh, we don't really know how dinosaurs slept. Seems most people believe that the smaller ones would be able to curl up on the ground, but it stands to reason the larger ones wouldn't be able to lie down for long. Horses sleep standing up because they cannot lay down for extended periods--their body weight cuts off circulation to areas of their body, which causes vascular injuries. And that's just for a horse that weighs about 1000 pounds. It would probably be even worse for a multi-ton dinosaur. I'm not saying it's not possible, just that it seems unlikely.
You might be right, but somehow I can imagine that biology would've come up with a way for them to sleep without standing upright. By evolutionary reasons you know. Horses or elephants don't really have natural enemies in the form of predators, but most of the big dinosaurs did. If they were standing up while sleeping, they would be a much easier target for theropods to spot, and I think evolution could have come up with a way for them to lay as close to the ground as possible to be less visible during sleep. Just a thought
Standing for sleep is actually beneficial for larger animals that would face predation. Typically animals that sleep standing up sleep more lightly than others, and they likely would not enter full REM sleep. This makes them more alert even while sleeping. And if they're already on their feet, they don't have to struggle with rising in order to flee.
Well I don't think fleeing will do them any good since they were probably slower than most theropods and could easily be caught up in a matter of seconds even if they were standing upright already when they noticed the danger. I presume hiding in the shade of the night would favor them better than standing up and having a better head start in a fleeing situation. But well it's just my thoughts and opinion, can't say for sure how it would end out. Also I'm quite sure REM sleep could be avoided biologically, in the way of being genetically designed (by evolution) to not enter REM too easily. Still, lighter sleep and more awareness would definitely be an advantage for them, I see your point. It would be nice to know for sure, maybe paleontologists will come up with an accurate explanation some day
Some were faster, some were slower. Depends on the species. There were also plenty of theropods that relied on smell rather than sight (t-rex is a good example), so darkness wouldn't necessarily help hide them. For every advantage in nature, there is almost always a disadvantage.
You know it funny, I know some people who own chickens. One day the had lifted up a tarp and bunch of mice scurried out from under it, the chickens chased after the mice and ate every single one ALIVE. So I guess chickens and dinosaurs are truly related.