There is the spell nap stack, which also cuts it down to two hours.
Just getting fatigue immunity works for non spellcasters, as that negates penalty for not resting.
What I find hilarious is that there is a PFS prestige award that let's you become a member of the Restful Pathfinder's Club. Through chatting, smoking cigars, and sipping brandy you get a full rest in six hours.
Couldn't anything that's less, but if it helps, resting just means not doing anything physically or mentally exerting, so you don't have to sleep.
making a Kineticist looking for a quick way to remove burn mid day. seems like 2 hours will have to suffice
I'm pretty sure that it's made clear somewhere that sleeping is something you do once every 24 hours. Reducing the time to rest to a standard action won't let you do it more than once a day.
The exception being the mythic rules, which do have abilities that allow you to rest more than once a day - at a minimal cost - and recover your daily abilities.
Frankly, if it were legal to just "rest" for 2 hours (or less) and act as if you'd done the once-per-day thing, you'd see people abusing it for wizards and other casters left, right, and center.