The sudden startled jolt awake sometimes experienced when on the verge of sleeping is known as a hypnic jerk, and is likely a vestigial reflex selected to help primates avoid falling while resting or asleep in trees.
fact
The sudden startled jolt awake sometimes experienced when on the verge of sleeping is known as a hypnic jerk, and is likely a vestigial reflex selected to help primates avoid falling while resting or asleep in trees.