I know if you find one perched somewhere you can but is it possible to sneak attack one if it attacks you from the sky?

If you hit it with an arrow before it sees you, yeah.

Or if you use a really long dagger. Yeah, that one was lame. . I don't have much luck shooting them out of the sky. It's so hard for me to hit a moving target, especially a flying one. Every time I miss, I'm like "dammit, another wasted arrow!"

A quick hint:

Many dragons sleeps on the ground. So you can dagger it.

I already killed one dragon with a one-hit dagger! The [censored] elder dragon died so fast I felt bad not fighting fairly.

Is it common to stumble upon a sleeping dragon? And is this usually at night or is it completely random?

Strictly at night. And often in burial stones, where you find words of power.... not sure if they sleep anywhere else.

If you have the shadow-warrior perk, (provided the dragon hasn't gotten agro with any other NPC's. You can't sneak attack an NPC that's in combat) you can force an impromptu sneak attack on the ground too

If you are a stone cold backstabber with the full sneak tree perked out and enchanted stealth gear, you can climb way around the long way 100 yards lr more above the dragon, sneak down, and then run up its tail and body and get a backstab in while standing on it. That will teach it to take cat naps.

can you sneak attack a dragon that is already attacking you if u pop an invisibility potion while its on the ground? im thinking of playing an assassin character with sneak and one dagger only soo if you cant i probably wont make a char like this haha xD

I can give initial backstab on dragons before the fight start, but i never could use shadow warrior in the middle of the battle when the dragons are on the floor.

As Heizen mentioned, they sleep. In addition the ones on the mountains near word walls usually sit perched on the wall stone. If you keep a invisibility spell up and keep sneaking you can just get higher on the mountain, drop down behind them, and then whack the tail for a 30x.

They get up and fly for a second after being struck, then crash into the ground and burn away.