Why do we forget dreams so quickly?

We forget dreams quickly because the brain usually doesn’t encode them as strongly as waking experiences, and any dream trace that does form is fragile right after waking.

A few factors likely contribute:

• REM sleep chemistry is different: Dreaming often happens during REM sleep, when some brain chemicals linked to alert, durable memory formation are in a different state than during waking.
• Brain activity is reorganized during dreaming: The networks involved in planning, monitoring, and making sense of experiences are not working the same way they do when you’re awake, so dreams may be less likely to get organized into a stable memory.
• Waking distractions overwrite the trace: As soon as you wake, attention shifts to sounds, movement, plans, and other input. That can crowd out a very fragile dream memory before it has a chance to settle.
• Dreams are often fragmented and unusual: Because they can be odd, discontinuous, and weakly connected to everyday memory cues, they’re harder to retrieve later.

So the short answer is: dreams fade fast because they’re usually encoded weakly, and waking attention can make them disappear before they become stable memories.