When we are asleep we go through different stages of sleep, one of this stages is REM (rapid-eye-movement) sleep. During this stage the body experiments rapid movement of the eyes, increased heart rate and brain activity similar to the one we have when we're awake. This brain activity produces very vivid dreams that the dreamer experiments as if they were real.
In this example Tao wakes up Don and he tells him that he was about to land on Mars after winning a fierce battle against flying jellyfish. We can see that <u>Don was experiencing a very vivid dream </u>and therefore, he was most likely in REM sleep
