- The wizard is pretty cautious... because they are a wizard and are squishy.
- The druid's player likes doing things that would be in character even if they aren't completely optimal, and likes the feeling of danger... because they are a cool Druid, and there is depth there. Right.
- We also have a monk who's frustrated by doing less damage than the other PCs (particularly the wizard). Yep. But monk'smonks get really neat abilities later in the game, and it makes sense that the player would feel a little underwhelming at lower levels.
Great. Actions have consequences. Party wakes up at 7 AM. Eats breakfast. Runs into some Orcs. Fights 3 rounds of combat (3 rounds * 6 seconds is 18 seconds later) and then wants to go back to sleep at 8 AM. So, you're the DM, what do you do?
- Monsters have friends. Ambush the party two hours later, when the orc party searches for the missing patrol the party just killed. This ruins the long rest. You don't have to always interrupt the sleep, but keep in mind that sleeping near the enemy at 10 AM is a bad idea in real life too. Also, a Nightmare Hag would love feasting on a party that only stays awake for an hour at a time.
- People have circadian rhythms. You simply can't fall asleep, the bright sun in your eyes and heavy breakfast YOU JUST ATE are preventing you from getting any shut eye. After laying there restless for an hour or so, what would you like to do? (and the rules support this (thanks Black Spike for finding PHB page 186):
A character can’t benefit from more than one long rest in a 24-hour period, and a character must have at least 1 hit point at the start of the rest to gain its benefits.
- Things don't stop because the players sleep. While the party is asleep, the BBEG conquers more land, takes more prisoners, and magical darkness spreads to the edges of another town. The big bad kidnaps one adventurer's relative, and kills five town guards. It might have been prevented, but you were asleep. The king is angry, he hoped people with your skill wouldn't be caught laying down on the job.
- Things happen at specific times "You can sleep, but you have to meet with Balalabad in an hour, and you are an hour and a half travel time from them at normal walking speed. Guess you don't want the reward for the gem you risked your life to steal from the giant for them."
- Let them split the party You have two players who want sleep, two who want to keep going. The monk and druid decide (maybe with the helpful DM comment, "just because they are sleeping doesn't mean you have to.") to keep going while the wizard and rogue snore. I don't often suggest splitting the party, but I don't think it would split the party. I think the rogue will likely decide the rest isn't worth not getting the XP and the possibility the monk and druid might not share their findings. Now, will the squishy wizard still think sleeping alone in the middle of a dungeon full of hostile monsters is the best course of actionsaction?
Why won't the encounters just happen the next session? What held off the other encounters? Why did one encounter and sleep take a whole session? I mean sleep, when you let them actually sleep is, "Oh, you want to sleep? Alright... You go to sleep." (giveGive time for someone to say if/who is on watch. If no one, then you'll have real fun.) "You are woken up an hour later to the strong hands (or tentacles) wrapping around your arms..."
Some of it might have to come from a session 0 like talk, where you ask them what their expectations are, tell them what yours are. In it assure your wizard player that, "I'm not here to kill your character. I'm here to make your character shine by giving them opportunities to pull through when the chips are down. To do that, there will be some risk of character death, but without the risk the reward isn't as meaningful. No one likes the story about the wizard who went to sleep. They like the story about the wizard who used their last spell slot to cast enlarge on the party's monk (who only had one hit point left) and the monk super puchedpunched a dragon in the face as the final blow in an epic battle. Trust me not to be gunning your characters down, and take a little more risk – it will be worth it."