Beyond the class features, racial traits, and feats that reduce the amount of sleep required for a long rest to 4 hours and the ring of sustenance that halves it I can't find any other ways to reduce the amount of time required.
Did I miss anything?
I was hoping to find a way because of a campaign i had an idea for where you would also be put into battles while asleep, so you would want to be able to have less sleep with the same effect to avoid those battles, not just as a way to make sleep less important. If there isn't a way I can homebrew something, but I would prefer to not homebrew anything if there is an existing way.