Some times at night i get a sinking feeling in my chest and it feels as though my heart stops. if i sit up it stops, but if i lie down it comes back.

See your doc: How much do you weigh? How many pillows do you sleep with? Do you snore? Do you have a family history of heart disease? High blood pressure? High cholesterol? Do you have a family doctor? When have you last seen him/her? I would like you to go and tell them about this and let them exam you. I have ideas and they will too, but if i tell you, my answer will get pulled! good luck, be well!