Causes for sudden body "jerks" while asleep or going to sleep? mild or sometimes strong involving whole body

Normal: This is a normal function of the body moving from wakefullness to sleep. As the body falls asleep it goes through mini-rem-type periods where the muscles slacken. These jerks are myoclonus twitches, or involuntary muscle spasms. Sleep soon follows.

Neurology: Basically it is nothing serious. It is your brain basically going to sleep.

Sleep "starts": Many people experience these sudden "starts" while falling asleep. They are a form of myoclonic jerking, and are usually in the range of normal. Other causes exist, which require more investigation: http://www.Ninds.Nih.Gov/disorders/myoclonus/myoclonus.Htm.

Restless leg syndrom: Based on your description, it looks like you might have restless leg syndrome. It is unclear to date how patient gets this syndrome but there are some known triggers and related systemic conditions like peripheral neuropathy, kidney diseases, iron deficiency... Caffeine and cigarette smoking worsening it. There are no cure but many medications could help relieved symptoms. You should see your doc.

It is normal: This is absolutely benign and normal experience and you should not worry about it. You need to watch if the nature is changed