Does the 5d8 apply to maximum hit points or current hit points?

I've always seen it ruled as "current" hit points, but that could be incorrect.

I always considered part of the tactics to this spell is to do light damage to a group then drop this spell on them because they have less "current health".

Always have to remember, it doesn't work on Half Elves.

Fey Ancestry

You have advantage on saving throws against being charmed, and magic can’t put you to sleep.

Also Elves....

I take this to mean that they fall unconscious for one minute, and then start to wake up? or they remain asleep until something gives them a reason to wake up, like damage or jostling?

So if I cast it on a couple goblins, effectively ending combat, so long as they rush to dispatch or restrain the goblins or how would you handle that?

They stay asleep for either 1 minute or until something attacks them/rouses them awake, plain and simple. So they're out for a full minute and you can take them out at your leisure at that point.

Will this always successfully affect at least 1 bad guy? The spell says that the remaining HP will affect the next lowest but it doesn't say that it WONT affect the first guy if he had more HP than what you rolled... just wondering because it's not clear.

From the description of the Sleep spell:

A creature's hit points must be equal to or less than the remaining total for that creature to be affected.

This is always fun to cast on flying PCs. 

For the benefit of those asking:

As Written: the spell specifies current hit points.

As Intended: A first level spell that is designed to deal with an inconvenient NPC or Non Fatally end a battle with already weakened enemies etc. 

In my interpretation, the spell still counts/effects Elf/Halfelfs because they only got advantage on charm spells, not imunity. Of course they are immune to sleep, so the spell is wasted. 

Undead on the other hand are not effected at all, so the slell doesn't even consider them. 

Great way to deal with that pesky level 14 or higher "Path of the Zealot" barbarian that is Raging Beyond Death, but is also at 0hp and has failed 3 Death Saves (unless they are also an elf of course).

Question,my group and I are in disagreement, they think the health pool that is rolled is a damage dealing pool ,I'm saying it's just a measuring tool to see who is effected,can someone please explain more in-depth?

So let’s, for ease sake, say you’re fighting 5 enemies, who’ve each taken some damage, which has removed some of their maximum 20 hp. You roll your 5d8 and you’re incredibly lucky, your total result is a 40. This is how the spell would go:

Bandit no. 3 has 5 HP - he’s asleep, your total is now 35. 
Bandit no. 5 has 8 HP - she’s asleep, your total is now 27. 
Bandit no. 1 has 13 HP - he’s asleep, your total is now 14. 
Bandit no. 4 has 15 HP - he’s not effected by the sleep spell. As this guy had the next lowest health in the lineup, the spell won’t effect anyone else targeted, because everyone else has higher health than him. 

The character sheet for an Elf states "...magic can’t put you to sleep" so I would suggest that a Sleep spell cannot send an Elf to sleep.

So that said, bandits no.3,5,1 are dead? I keep wondering how is the damage applied because I can't seem to understand it. Also the npcs need to have damage dealt beforehand for the spell to affect them? 

No, Bandits no. 3, 5 and 1 are asleep, not dead. They still have the same hp as before they were sent to sleep. There is no damage dealt. It's a status effect, not a damage spell.

You don't necessarily need to have damaged the target(s) before using the spell, but the less hp they have when you use the spell the more creatures you can effect. 

So, there's no wisdom saving throw? Pretty awesome, especially at higher levels.

does this affect the caster