Why do we forget our dreams?

During REM sleep, when most vivid dreaming occurs, levels of the neurotransmitter norepinephrine, which helps encode memories, drop sharply, and the brain regions responsible for storing long-term memories are relatively inactive. As a result, dreams are stored only fleetingly, more like short-term impressions than durable memories. If you wake directly from a dream you may recall it briefly, but without deliberate rehearsal the memory fades within minutes as the brain shifts into its normal waking mode and new sensory input crowds it out. The hippocampus, crucial for turning experiences into lasting memories, communicates poorly with the rest of the brain during dreaming. This is why keeping a dream journal and writing dreams down immediately on waking is the most reliable way to remember them.