Use Max HP or Current HP?

Tim Shorts asked this question about sleep yesterday - in D&D 5e, do you use the current HP or the max HP of creatures to determine how many HP of creatures are subject to sleep?

Here is the simple guide I'd use for all such effects, in all D&D-compatible systems.

Use Current HP when determining what happens to that creature. A sleep spell worth 7 hp will put you to sleep if you have 7 HP or less, no matter what your max is. An AD&D-style Power Word: Kill spell will kill you if your current HP is exceeded by the spell's effects.

Use Max HP if HP are used to determine an offensive effect. An 88 HP dragon would do an 88 HP of damage breath attack even when it's down to 1 HP.

Pretty simple. Effect on you? Current HP. Your effect on others? Max HP. Done. This approach opens up a lot of interesting tactics and makes a weaker fighter more vulnerable to more spells, and a weak monster still a full-strength threat. That's win-win.