Based on a true story:
The Knucklebiter Gnat must be defeated before attempting to go to sleep. If the character fails to destroy it, it will harass them all night and instead of getting the benefits of a long rest, they will gain one point of exhaustion.
It's a fun little thing, I think. Everytime the characters find it (investigation/perception vs. its Stealth) they get to make 1 attack against it. If it is not killed, the character may try again.
Add more Gnats until the characters give up and simply don't care.
Next day hit them with the exhaustion. From now on, you can force your party to have to fight Knucklebiter Gnats now and again because it'll be horribly annoying not to and, as people in real life, they'll get increasingly irritated and extreme in their measures to try and hunt those little monsters down.
Ideal for swamp, temperate and desert climates.
Bonus for storytelling: After a night with one of those around, narrate every failed skill check or attack with "You cannot concentrate due to the horribly itchy bites left by the Knucklebiter Gnat. Being this distracted, you fail to spot the trap."
You can totally adjust that. The point really is to be exceedingly annoying in it's base setup. But maybe I also just have way too many crits in my parties.