Have you ever felt your body jump just as you fall asleep? Sometimes you also feel like you are falling. This is called a hypnic jerk. About 60 to 70 percent of people have it, so it is normal.

Scientists think the signal comes from a deep part of the brain. Being very tired, drinking coffee or feeling stress may make it happen more often.

Just as you drift off to sleep, your body may suddenly jerk, sometimes with a brief feeling of falling or missing a step. This is called a hypnic jerk, or sleep start, and it is experienced by around 60 to 70 percent of people. It is considered a normal event linked to the change from being awake to being asleep.

By recording which muscles move and in what order, researchers have found signs that the signal begins in deep parts of the brain rather than in the cortex. Popular stories say the jerk comes from our ancestors falling out of trees, but there is no scientific proof. Tiredness, caffeine and stress may make jerks more frequent.

The sudden jolt many people feel as they drift off to sleep, often with a brief sense of falling, is known as a hypnic jerk. Studies estimate that around 60 to 70 percent of people experience it, and sleep disorder classifications treat it as a normal phenomenon of the transition between waking and sleep.

In 1959, the British researcher Oswald linked these jerks to the process of falling asleep, and later sleep recordings showed that they tend to occur in the earliest stage of sleep. To locate their source, researchers have examined the order in which muscles activate. In 2013, Chokroverty and colleagues studied ten people whose jerks were strong enough to cause insomnia and found patterns resembling a startle reflex or spreading from top to bottom.

A 2016 study supported the idea that the jerks originate below the cortex, possibly in the brainstem. Popular explanations, such as a reflex inherited from primate ancestors who feared falling from trees, remain untested. Although usually harmless, frequent jerks can disturb sleep, and similar-looking movements may signal other conditions such as some forms of epilepsy. The 60 to 70 percent figure is based largely on self-reports and varies between studies.