Mettle + Negation

[Jack Y.]

Howdy! I'm back at it again at the ARC forum with a hot new question.

The crux of my question is does Mettle allow a character to defend against a carrier attack with a non-instantaneous effect?

Example:

Gargathor the fighter has a 30 Sleep coming their way, and their armor's running low. They want to mettle it, they have the body points available.

Are they able to Mettle this?

Per Defenses on page 32,

Only one defensive ability can be called at a time, if you defend against part of an attack, you defend against all of it.

But wait! Mettle only works on non-instantaneous effects!

Mettle*
Martial
Daily
Prereq: 1 per 30 Martial XP
When struck by an effect with a non-instantaneous duration, the character may call “Mettle” and expend 20 Body Points. This reduces the duration of the effect by 5 minutes, which may immediately negate it.

The question I have is with the effect. Damage is understood to be instantaneous, the Sleep effect that it is also delivering would be completely negated if reduced by 5 minutes.

In this circumstance, what happens/what is the order of operations?

Thank you for your time and consideration,

Jack!