Activation synthesis theory

Random activation: during REM sleep, the brainstem sends out bursts of random electrical signals. These activate different parts of the brain, including areas for memories, emotions and senses.
Sensory blockade: information from the outside world is blocked from entering the brain during REM, so the brain works only with its own internal activity.
Movement inhibition: signals to the muscles are blocked, so the body is paralysed and we do not act out our dreams.
Synthesis: the higher brain (the cortex) tries to make sense of the random signals by weaving them into a story, using stored memories. The result is a dream.

It is based on biological evidence: EEG and brain-scan research shows the brain is very active in REM.
It is more scientific and testable than Freud's theory.
It explains why dreams are often bizarre and jump between scenes: the signals are random.
It explains why we cannot move in dreams (movement inhibition) and why outside sounds rarely reach us (sensory blockade).

It suggests dreams have no meaning, yet many dreams link to real worries and recurring dreams happen, which random activity struggles to explain.
Dreams can also happen outside REM sleep, so REM activity cannot be the whole explanation.
It is reductionist: it reduces a rich experience to brain signals.
It describes how dreams happen but says little about why the story has the content it does.