A hypnic jerk, or sleep start as it is otherwise called, is an involuntary muscle movement made during sleep, or while coming into or out of sleep. They often occur in the first stage of sleep, the transition between a wakeful and sleeping state. These movements are generally only noticed when they are forceful enough to wake someone up. Many people may experience hypnic jerks but have no idea because not all movements are strong enough to bring a person back to a wakeful state.

Most people also experience very slight jerks during REM sleep, when the body has lost virtually all muscle tone (thanks to REM atonia, the paralysis that kicks in at this time of night) but may be attempting to act out the activity of a dream.

Hypnic jerks that take place while a person is waking up are less frequent and are known as auditory sleep starts because they are frequently accompanied by a perceived shattering or cracking noise.

Hypnic jerks are relatively common and are typically no cause for concern or embarrassment. They are estimated to affect roughly 10% of the population every night, but up to 80% of people occasionally experience them (think of anytime you have seen a student in class or in the library trying to fight off sleep and suddenly jerk into a wakeful state).

In almost all cases hypnic jerks pose no immediate danger or health concern, except for the fact that they can disrupt the sleep of a bedmate. Besides being annoying, if this happens frequently the bedmate's alertness in the following days could be compromised, which can pose serious risks.

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