It's a perfectly normal feeling and something shared with the majority of all humans on the planet. Here's what Wikipedia says about it:
Hypnic jerk, also called as hypnagogic jerk, is a kind of sleep disorder that can be caused by anxiety, caffeine, a dream, or discomfort of sleeping. Basically, hypnic jerk is the feeling when people experience a sudden muscle twitch and feel like they are falling while sleeping and sometimes dreaming. About 70% of people have experienced hypnic jerk.
There is not completely accurate explanation of this phenomenon, but most experts agree that as muscles begin to slack right after one falls asleep, the brain senses these relaxation signals and misinterpret as one is falling down. Then the brain will send signals to muscles to keep the balance.
Studies also showed that there is a correlation between the frequency of hypnic jerks and anxiety. Fatigue, discomfort of the bed, highly intense exercise and stress might cause people to jerk during sleeping.
There are several ways to prevent the hypnic jerk:
Follow good sleep hygiene, develop a routine and follow it every night.
Watch what you eat before sleeping.
A magnesium, calcium or iron deficiency may be to blame. First try to improve your diet with fresh fruit and vegetables. Or try a supplement, to provide those minerals and vitamin C.
Heavy exercise too close to bedtime can have an effect. Try to limit heavy weights or cardio sessions in the evening. Conversely, a gentle walk an hour before bed may help.
Smoking or drinking will reduce the quality of your sleep. One drink before bed may help you to fall asleep and avoid the hypnic jerk; however, it will reduce the quality of your later sleep. If the jerk is severely debilitating, then this may be a way to get things under control.
Meditation can help relive negative feedback loops that will make you dread going to bed.
Maintain a positive outlook, and get some tips of relaxation or yoga. The techniques taught in even the most basic class may be enough to allow you to get your mind in a state where sleep comes naturally.
A relaxation CD played at night may be the answer. It will give your brain something to do instead of worrying and may quickly become an essential part of your sleep routine.
If the sleeping disorder is having a negative effect on your life then see a doctor, who may prescribe some medication to help you get that much needed sleep.