Sleep is based on nothing more than a save vs spell.

nt

That's clearly false, as successive sleep attempts have a lower success rate the more times you fail.

I still think it's easier to land sleep than any other spell.

Some spells (e.g. plague) are much easier to land against certain enemies, because they are related to con.

This is correct.

The first sleep is a normal spell save; it gets penalized from there with additional tries.