Can you control what you dream?
Scientists have begun a study to see if it is possible to influence what we dream about.
They have designed a smartphone app which plays sounds such as birds in woodland or waves lapping against the shore.
The idea is that the noises should direct pleasant dreaming.
Prof Richard Wiseman, who is launching the trial as part of the Edinburgh International Science Festival, wants thousands of people to take part.
Rather than recruiting people directly, the 'mass participation' trial relies on members of the public volunteering.
The sounds are played around 20 minutes before participants say they want to wake up.
The Dream:ON app monitors movements to sense when the sleeper has started dreaming. It then plays the sounds.
When it senses the person has stopped dreaming, the alarm goes off. The person is then asked to send a description of the dream to the 'Dream Catcher' database.
Prof Wiseman said: "Getting a good night's sleep and having pleasant dreams boosts people's productivity, and is essential for their psychological and physical well-being.
"Despite this, we know very little about how to influence dreams - this experiment aims to change that."