If the ビクン action refers to the rather reflexive twitches when entering to sleep, hypnic jerk appears to be the word for that, as Harumi answered.

By the way, ビクン here is an onomatopoeic expression for such jerk of body muscle(s). Twitch is more towards ピクン/ピクピク/ピクッ. It simply depends on the degree of muscle movement, that I understand.

専門用語としては、「myoclonus」と言います。

http://www.wisegeek.com/what-is-myoclonus.htm

Myoclonus is a term used to describe the jerking or twitching of a muscle. Healthy people experience myoclonus on a regular basis and it can also be the symptom of an underlying medical condition which requires medical attention. This involuntary muscle movement can take a wide variety of forms, appearing in any muscle in the body at any time, depending on the underlying cause of the myoclonus.

http://house.wikia.com/wiki/Myoclonic_jerk
A myoclonic jerk is the brief, involuntary twitching of a muscle or group of muscles. It may be caused either by a sudden muscle contraction, or a sudden lapse of contraction. This happens when a person is on the verge of falling asleep, and suddenly have a sensation or feeling that they are free falling through the air.

http://emedicine.medscape.com/article/1176055-overview
Myoclonic seizures are characterized by rapid, jerklike movements that can affect the face, limbs, or axial musculature. Most families are familiar with hypnic jerks; that is, a sudden jerk that jolts one awake while falling asleep.
http://www.columbiaepilepsy.org/teens/docs/Sleep.pdf
http://www.aboutkidshealth.ca/En/HealthAZ/ConditionsandDisea...
http://medical-dictionary.thefreedictionary.com/Myoclonic je...
http://www.buzzle.com/articles/twitching-while-sleeping.html

In medical terminology, the involuntary twitching of a muscle or a muscle group is referred to as myoclonus. A hypnagogic jerk, which is also referred to as a sleep start or a hypnic jerk, refers to involuntary muscle twitching that may occur when one is just about to fall asleep. They start to occur during the transition from the state of wakefulness into the state of sleep. These are experienced by a majority of people and are considered to be harmless. However, these must not be compared with frequent episodes of involuntary muscle movements that may occur in the other stages of sleep. Sometimes, neurological disorders could also be the underlying cause of twitching while sleeping. Given below is some information on the causes of involuntary muscle twitching at night.
Read more at Buzzle:
http://www.buzzle.com/articles/twitching-while-sleeping.html