Beginner THM help- using Sleep

[Terrini]

I play heals and the first time I really noticed Sleep being so helpful was in my first run of Haukke Manor where the BLM slept all of the non targeted mobs and it was such a breeze to heal XD It's a whole lot easier too when you get the trait that allows you to sleep multiple mobs at once (I forget if it was Level 26 or Level 31, but I remember that I really wished it was a level sooner because it helped so much on the nearest class quest).

When I DPS I usually BLM though, so I look at the party make up, if the tank is a PLD and the other DPS are single target Melees, I'll sleep. WAR have more AoE damage moves, SMN tend to spread the DoTs around, BRDs have a lot of AoE damage, as do NIN, DRG and MNK not so much. So I sleep and single target when that's the make up of the group... You probably get the most flak from speed runners that were expecting you to just AoE down a huge pull. So if they're doing big pulls, just AoE and they'll be happy. When you need to swap to single target to regain mana, go ahead and sleep in the short down time.

[Delta041]

If your tank is taking the time to mark targets with numbers and doing slow pulls, go ahead and sleep the ones further down the chain. Otherwise just focus on DPS.

[Niwashi]

When I DPS I usually BLM though, so I look at the party make up, if the tank is a PLD and the other DPS are single target Melees, I'll sleep. WAR have more AoE damage moves, SMN tend to spread the DoTs around, BRDs have a lot of AoE damage, as do NIN, DRG and MNK not so much. So I sleep and single target when that's the make up of the group...

This is a good approach, but it requires a good understanding of all the different classes/jobs, what sort of skills they have as well as when they get them. It's one of the reasons I advocate playing all the classes in the game a fair amount, even if you plan to focus on a single favorite one for endgame content. Having played them all gives a better understanding of how they work than simply having played alongside them all, and that understanding makes people better at coordinating with the rest of a team.

... a lot of AoE damage, as do NIN...

An example of the "when they get them" part being important: NIN only have useful AoE at or near endgame. They have none at all until 35, and then only a single AoE that shares a 20 second cooldown with all other ninjutsu skills until level 42 when they get Death Blossom, the only spamable AoE for the class. I remember a Haukke Manor run a while back where the tank was gathering up huge groups of mobs apparently expecting us to AoE them. I was a ninja, but even if I'd unlocked any AoE skills (which I hadn't yet) I'd have lost them to the level sync there.