“dying in your sleep” really could be among the worst possible deaths

in your sleep you can be having a nightmare

in a nightmare, “how bad an experience you can be having” is limited only by the bounds of your wildest imagination

dying in your sleep is basically dying in the Matrix if the Matrix machines hate you and want to torture you to death

(source: i have been having sleep apnea and woke up VERY badly just now. Argh)