Could be hypnagogic hallucinations.

Excerpt:

Sometimes you may see entire scenes. They are usually incomplete (if they were complete they would be a dream) but are quickly becoming whole. They happen in the same way as the memory footprint. As your consciousness dips into the subconscious mind it starts picking up the scene that it is building and you may see parts of it fade in and out. This is usually the last stage of hypnogogic hallucinations before you enter a dream.