Someone asked: if I dream of doing something sinful, such as dreaming of killing an animal, does that count as real demerit for me?

Dreaming of killing an animal has no real effect on the human body, because the animal in the dream doesn't actually exist — it's only a feeling that startles the mind, causing fright or an uneasy sense. The unwholesome can use this opening to induce some influence, but it carries no serious consequence, because the human realm is the realm of forming causes, unlike other planes of existence, which are realms of experiencing results. A cause that truly bears fruit must arise from the intention of the human body while awake.

The Human Realm Forms Causes; Other Realms Experience Results

Even after death, in another plane of existence — a subtle body, a Brahma realm, or a formless Brahma realm — actions there can still somewhat affect the experiencing of merit or demerit already accumulated. In the Tavatimsa heaven, for instance, listening to the dhamma can extend the duration of experiencing merit's fruits, while excessive indulgence in sensual pleasure can shorten that duration, leading to rebirth in a lower heaven. But these only affect the experiencing of merit or demerit already present — quite unlike the human realm, which is where wholly new causes are formed directly.

The human realm thus acts like a junction. After death, one experiences the fruits of merit and demerit already accumulated until they're exhausted — perhaps being reborn in a Brahma realm for millions of years. As that merit runs out, one descends through the various heavens as a subtle divine body, level by level, until merit is no longer sufficient to remain in heaven, and one is reborn human again to experience the fruits of demerit — or even when demerit isn't enough to fall to hell and merit isn't enough to rise to heaven, one must be reborn human once more to begin forming new causes again.

Intention Determines Karma, and Dreams Involve No Intention

It has been said that intention is karma itself. While a human is asleep and dreaming, there is no intention present, so the standard that applies to the waking human body cannot be applied to the dreaming body. Whatever the dreaming body seems to do is merely something relayed to the waking human mind — it carries no real consequence, only stirring the mind with fright, giving rise to worry and inducing a small, minor trace of demerit at most. There's no need to dwell further on such things. It is enough that, while human, we act with wholesome intention and genuinely refrain from killing living beings; whatever lies beyond our control in a dream has no real effect on us.

Quite Unlike Actually Killing, Where Life-Energy Is Truly Drained Away

Every time one actually kills a living being while awake, life-energy is genuinely drained away. When such an act is committed, a sphere of demerit born from destroying life forms and begins drawing our life-energy into itself, much like a black hole pulling everything in. Our life-energy is weakened according to the size of that sphere of demerit — whether the creature killed was large or small — shortening our lifespan accordingly, and carrying karmic results that continue to weaken our future life as well, becoming a completed pattern that bears fruit further down the line. This is entirely unlike dreaming, which involves no intention and no real living being at all.