A dream can feel incredibly vivid while it is happening.
Then you wake up.
For ten seconds you remember nearly everything.
A minute later, the whole thing may be gone.
That happens because dream memories are formed under unusual brain conditions and are often not stored very securely.
Memory works differently during sleep
During sleep, especially REM sleep, the brain’s chemistry and activity patterns differ from waking conditions.
Systems involved in making stable long-term memories do not work in exactly the same way.
The dream can feel real while it is happening without automatically becoming a durable memory.
Waking creates a flood of new information
The moment you wake, your brain starts processing the room, the alarm, the time, your phone, the weather, what you need to do, and everything else.
That new information can quickly interfere with a fragile dream memory.
The dream did not necessarily disappear instantly.
It got crowded out.
Why do some dreams stay with us?
Strong emotion helps.
So does waking directly from the dream.
If you wake during or immediately after a vivid dream and think about it before doing anything else, you are more likely to remember it.
Dreams that feel strange, frightening, funny, or personally important may also get more attention.
Attention gives the memory a better chance to stick.
Does writing dreams down help?
Yes.
Writing down even a few details immediately after waking can preserve information that would otherwise disappear.
Often one remembered detail triggers another.
A place leads to a person.
The person leads to the bizarre part with the talking refrigerator.
Soon more of the dream comes back.
Why do we sometimes remember a dream hours later?
A cue can bring it back.
A smell, place, phrase, or event during the day may overlap with something from the dream and reactivate part of the memory.
The memory was weak, not necessarily completely erased.
Do people who remember dreams more often dream more?
Not necessarily.
They may simply wake more often during dream-rich periods or pay more attention to dreams after waking.
Dream recall and dream production are not the same thing.
So when a dream vanishes almost immediately, you did not imagine the forgetting.
The memory was never filed very securely in the first place.
Your brain basically wrote it on a sticky note during the night.
Then morning happened.
A related piece of human behavior appears in Why do we dream?, where the brain handles another everyday judgment or response.
And that’s the short explanation.
Related explanations: Why do we dream?, Do dogs dream?, and Why do we get déjà vu?