Sleep starts are so common that they almost don’t count as a sleep disorder. Most have experienced them – a sudden, often violent, jerk of the entire body or one or more body segments that occurs upon falling asleep. Sleep starts are also known as hypnic or hypnagogic jerks. Somewhat related are: a visual sleep start – usually a sensation of blinding light coming from inside the eyes or head and an auditory sleep start – a loud snapping noise that seems to come from inside the head.
If the fluttering of the eyes during stage R sleep are a visible indication of sleep in the waking world, so also hynic jerks are an intrusion from the waking into the dream world.
Contributing factors that can lead to sleep starts include stress, exercise before bed, and caffeine or other stimulants. Patients that suffer sleep starts often have an iron deficiency and/or a urea buildup. They are also potential side effects of some antidepressants and antihistimines.
At times, many jerks may occur one after another. They can be frequent, intense, and repetitive. Intense or frequent sleep starts may lead to a fear of falling asleep. In most people, they only occur from time to time. Sleep starts are particularly common in young children, although they happen throughout life. The mechanism is not known, but anatomists suspect it is a playing out of a struggle between the part of the brain that promotes waking and the part that promotes sleeping (the VPLO).
The human mind is so narrative-driven that it can make up stories or scenarios and incorporate them into dreams. People sometimes report dreaming of falling or tripping during the hypnic jerks.
They are rarely treated, but if they get bad enough treatment options include leg exercises, iron supplements, and in bad cases, dopamine agonists similar to those used for restless legs syndrome, although pregnant women can’t have dopamine agonists.