Well, there's a name for this...
It's called Hypnic jerk. A hypnic jerk, hypnagogic jerk, sleep start, or night start, is an involuntary myoclonic twitch, (an involuntary twitching of a muscle or group of muscles), which occurs during hypnagogia, (the transitional state between wakefulness and sleep, i.e., the onset of sleep), often causing them to awaken suddenly. Physically, hypnic jerks resemble the "jump" experienced by a person when startled, often accompanied by a falling sensation. A higher occurrence in people with irregular sleep schedules is reported.
Ok, so now we have a 'scientific' name for it, but what causes it? Why do we do this?
Well, experts are still not completely sure why the body does this, although the occurrence is well known and has been well documented. According to the American Academy of Sleep Medicine there are a wide range of potential causes, including: anxiety, caffeine, stress, and strenuous activities in the evening. Close to 70 percent of all people experience this phenomenon just after nodding off, according to a recent study at the Mayo Clinic. Most experts agree that this is a natural part of the sleeping process, much like slower breathing and a reduced heartbeat.
The consensus among researchers is that, as the muscles begin to slack and go into a restful state as sleep is entered, the brain senses these relaxation signals and misinterprets them as indications of falling. The brain then sends signals to arm and leg muscles in an attempt to regain balance. This misinterpretation that takes place in the brain may also be responsible for the "falling" dreams that accompany the falling sensation. These "dreams" are not really normal dreams, as they are not produced from REM sleep, but rather more like a daydream or hallucination in response to the body’s sensations.
While this phenomenon happens to most, studies have recently begun to link some occurrences of "Hypnic jerks" to sleep anxiety, fatigue, and discomfort. People who are having trouble sleeping or cannot get comfortable in bed appear to experience the sensation more often throughout the night. It is especially more common with people who are trying to fight falling asleep or have deprived themselves of sleep for more than 24 hours.
Researchers believe that the lack of sleep from sleep anxiety or sleep deprivation confuses the muscles and the brain. The muscles continually attempt to relax and shut down for rest, while the brain remains awake creating continued "misinterpretations" of falling or loss of balance.
Scientists and researchers continue to study sleep twitching and jerking in a small capacity, but state that the sensation is completely normal for our bodies and is of little medical significance. Our bodies go through several procedures of shutting down and preparing for an extended period of rest. "Hypnic jerking" is just one of them. It does not appear to cause damage to the body and poses no danger to its physical wellbeing.
Hypnic jerks can occur in anyone. These jerks or sleep startles normally occur at the onset of sleep, rather than at the offset. During an epilepsy and intensive care study, the lack of a preceding spike discharge measured on an epilepsy monitoring unit, along with the presences only at sleep onsets, helped differentiate hypnic jerks from epileptic myoclonus.
According to another study on sleep disturbances, hypnic jerks occur during the non-REM sleep cycle and is an “abrupt muscle action flexing movement, generalized or partial and asymmetric, which may cause arousal, with an illusion of falling.” Hypnic jerks are more frequent in childhood with 4-7 per hour at the age ranging from 8 to 12 years old, and it decreases toward 1-2 per hour at 65 to 80 years old.
Source(s): wikipedia, sleep.com/