Does DR apply to damage from Diehard?

Question

You can take a move action without further injuring yourself, but if you perform any standard action (or any other action deemed as strenuous, including some swift actions, such as casting a quickened spell) you take 1 point of damage after completing the act.

Emphasis mine.

Is this damage subject to DR? I'm actually quite unclear as to what DR even applies to in general, other than clear exceptions like "Spells, spell-like abilities, and energy attacks (even non-magical fire) ignore damage reduction." Like... is falling damage reduced? And so on. This feat is my primary concern for this question however.

Answer 1

No. Read the Damage Reduction rules. It only protects against attacks.

Some magic creatures have the supernatural ability to instantly heal damage from weapons or ignore blows altogether as though they were invulnerable.

The numerical part of a creature's damage reduction (or DR) is the amount of damage the creature ignores from normal attacks. Usually, a certain type of weapon can overcome this reduction (see Overcoming DR). This information is separated from the damage reduction number by a slash. For example, DR 5/magic means that a creature takes 5 less points of damage from all weapons that are not magic. If a dash follows the slash, then the damage reduction is effective against any attack that does not ignore damage reduction.

Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury poison, a monk's stunning, and injury-based disease. Damage Reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact...

Attacks that deal no damage because of the target's damage reduction do not disrupt spells.

Diehard, falling, etc. are not attacks. Therefore DR does not apply.

Answer 2

mxyzplk has it right. However, many tables rule that most normal sources of damage qualify as attacks, even when they're coming from sources other a direct physical assault. A more specific RAW reason the damage from Diehard (and falling damage, which I only today discovered my group has been handling wrong by allowing DR to apply as though it were bludgeoning damage) isn't prevented is because it is untyped damage (emphasis mine):

Spells and effects that do untyped damage are pretty rare in the Pathfinder Roleplaying Game, since these spells are quite powerful since their damage can't be stopped by any form of immunity, resistance, or damage reduction.

Most physical damage sources do bludgeoning, slashing, or piercing damage, all of which can be stopped by DR. Most magical damage sources do elemental, sonic, or force damage, all of which can be stopped by resistances. Untyped damage cannot be stopped. Most feats that allow you to damage yourself for an effect do untyped damage so that you can't use DR or resistance to get around the cost of the effect.