First of all, sleep relies on what lvl your sleep skill is AND how much m.def the target has, more mdef-->less chance of landing sleep. Also level difference may or may not have something to do with it.
When talking about stuns its a bit different. Skill level applies here too, But tests ive done shows Mdef doesnt. Your stun always has a certain % chance of landing on your target at a certain lvl stun works pretty much 100%. When being about same level, rate is less and so on..
As an example when fighting at the PvP arenas stunning ppl lvl 35 and below when being 38 was a piece of piss and worked almost every time for the whole 8 seconds (stun hardly ever broke), on the other hand when challenging players lv 40+ things are different and stun success rate was more like 1/3 and when succeed
and it broke on several occasions.
It also has to do with different classes being more stun tolerant if you see my meaning, and it has to do with the basic race stats like DEX etc. (dont know which 1 comes in favour when getting stunned though) Ive experienced DE fighters have less resistance to stuns than most other classes.
Stun sticking has to do with the level of the skill vs the mob's MDef, as well as the con of the mob (meaning I stick my pre-40 stun attack on a white mob as often as I do my higher, post-40 hammer crush, if not a bit more often)...same with sleep or any other "magic" skill...the damage done and success rate is based on your MAtk and skill level and player level compared to their MDef and level.
Against equal mobs, though, sleep has a much higher success rate than stun...and non-damaging stuns (like shield stun) have a higher success rate than damaging stuns like stun attack and hammer crush.