Kindness does more than improve mood in the moment, it appears to have a measurable calming effect on the body's stress response. Acts of giving and helping have been associated with lower levels of stress hormones and a steadier heart rate compared with neutral or self-focused activity of similar duration.
One explanation is that kindness shifts attention outward, away from a person's own worries and toward someone else's immediate need, which briefly interrupts the internal rumination that often drives stress in the first place. Helping someone else can function, almost incidentally, as a short break from one's own difficulties.
This does not mean kindness is a substitute for addressing the source of stress directly, but it does suggest that a small, genuine act of helping, offered even on a difficult day, can provide real physiological relief rather than simply feeling like a pleasant distraction from what is actually going on.