Why does your body suddenly 'jerk' when you're about to fall asleep?

It's a neural misjudgment where your brain can't distinguish between 'falling asleep' and 'dying suddenly'! This phenomenon is medically known as a 'Hypnic Jerk'. As you transition from wakefulness to sleep, your heart rate slows, breathing becomes more relaxed, and muscles loosen, with the nervous system gradually handing over control to the sleep-dominant regions. However, if you're in a state of excessive fatigue, high stress, or have consumed too much caffeine, a 'signal error' can occur in the motor nerve area of the cerebral cortex. At this point, the brain mistakenly interprets this muscle relaxation and slowing heart rate as a danger signal that 'the body is losing vital signs or falling from a height'! To save you, the brain instantly issues a strong motor command, causing muscles to contract explosively and 'jerk you awake' to confirm safety!