Dogs are thought to dream in a very similar fashion to humans. They progress from quite a light sleep stage where they can be woken relatively easily to a deeper stage of sleep (similar to the human REM stage). It is during this stage that they begin to dream. Whining, yelping, kicking, twitching and rapid eye movement are all common during deep sleep. It is assumed that during this stage dogs are dreaming or in some cases having nightmares. It is perfectly normal and you should not try to wake your dog as they require several hours of uninterrupted deep sleep just like humans.