Sleep paralysis
When people are dreaming, their muscles do not move, even if in their dream they are moving. This is a natural process which prevents that people actually perform the movements they do in their sleep. In general, people are not aware of this inability to move, as it usually stops as soon as they wake up. People suffering from sleep paralysis wake up and find that they are unable to move. This is because the paralysis has not stopped. Usually it lasts for only a few minutes, and people have said they feel a "presence" or "ghost" during it.