Sleep Paralysis is a horrible & Frightening experience,when you are fully awake but completely paralyzed. Almost 20 % to 40 % people experience this once in their life time. I am the lucky one as I am experiencing it since my childhood. It has now reduced to 3-4 times a year as, some how, I have developed a self defense mechanism.
Number of people, who experienced this, also reported foul smells and sounds of approaching foot steps. However I have never felt any of these. Never the less the feeling that someone is sitting on my chest is the same as reported by many.