There is actually a build of up of chemicals as you use the brain, whilst you sleep you body removes these chemicals, the removal of said chemicals activates nurons in your brain connected with memories, when a memory is active it can be seen "behind the eyes" as a dream, sometimes if youve had a stressful day your brain will start by storing the memories of the day choosing which bits to store, whilst it works this out it moves old memories around and juggles them in your mind till its happy and done. one dream can last maybe 2 seconds before the next begins you can have some dreams that last hours, some people believe that training your brain to remember the prominent colours of dreams can predict the future. It's intresting stuff really