Alchemists Fire will certainly do the trick.
I mean it's obviously a joke (that I must stress technically would do the trick), but what it boils down to is disturbing sleep is easily done through any number of mundane means. Noise, splashing water on the target, undercooking their food to make their stomach upset, grinding some poisonberry into their drink, rubbing poison ivy on them, etc.
You could always take the child approach and keep waking them to ask them questions or to get their help with something.
Some casters can use familiars to constantly bug the sleeping player. Unseen servants can harass them. Mage hand, summoned creatures, Ranger animal companions, and even just regular creatures somebody performs successful animal handling checks with.
Or you can go the really extreme route and scout for enemies. Once you find a group of goblins, highway men, orcs or other creatures with intelligence, you can strike up some barter and hire them to harass the camp and prevent that player from getting a good night's rest. Alternately you could lure hostile creatures into the camp to do the same thing.
There's literally no limit to the number of ways you can interrupt sleep. Just think about times you've been interrupted in your sleep and apply them in game.
I'm going to relate a scenario that I used to give you an idea of what I mean.
Objective: Cause a 1 hour interruption in the long rest.
So to do this I start by looking for a sufficient enemy to achieve my goals. I find a tribe of orcs within 10 kilometers of the camp. I know that if we leave them be, they'll ignore us and probably not even know we were here, but that's not what I want. So instead I let myself get spotted by a sentry of theirs, and flee through the forest back to my camp, making sure to leave clear markings where I'm going. Now, I know where I'm headed so I'm going to get quite a lead. As part of my plan, I happen to drop a map that has our camp marked, as well as other camps we've used on our journey this way.
Once I get back, I burst into the camp screaming that orcs are coming. The party, now alerted, stands to and begins hastily strapping on gear and getting weapons. The orcs will either crash in and we have an encounter, or I managed to lose them. Either way, we get a tense 10 minutes of waiting for any sign of the orcs, followed up by another 5 to 10 minutes of searching for any sign of them coming. When it was apparent they were not coming, everybody turned accusatory eyes to the antagonist who interrupted their rest.
The sleepy characters grumble about having wasted sleep and begin peeling everything off again. Just as they're about to settle down, I mention that the orc party is rather large, and while it might suck, we should probably move camp. Then I reveal I must have lost my map while scouting because I can't find it anywhere, and the orcs might have it. Sighing, I point out that we probably need to move camp or we're going to continue getting attacked.
If they don't choose to react to the orc threat: Five minutes after they're settled, screaming that you can hear incoming wargs repeats the earlier process. Orc messengers and scouts sometimes travel on wargs to get around quickly, so the group should be reacting as if there's a threat again. This accomplishes the 1 hour break in the long rest needed.
If they do choose to move campsites: Tearing down and setting up a camp takes time. Between strapping on armor, striking tents, readying the horses, travelling to a new location and setting everything up again, at least an hour will have passed.
There you go. Either way the long rest has been interrupted.