Drained and exhausted, you are finally in bed sinking into what will be a deep sleep, when your body starts ‘falling’ and it suddenly jerks to stop that ‘fall’.
You will of course wake up immediately and wonder where it came from.
And no matter how many times happens, you never get used to it, it never feels less abrupt, and it never gets any better. The falling sensation can even be terrifying, which ends with an involuntary, sudden, and shocking jump.