Distraction from Lucid dreaming

As I wake up at 5am and get ready to do WILD, just before I get into paralysis and while I'm feeling like my body isn't there, something happens that snaps me out of concentration and my relaxation.

My left or right arm randomly jolts in the air like some puppet master has my arm on a string and sometimes hit myself on the chest or even the face that ends snapping my focus to get ready for paralysis.
I have no idea how to stop this from happening but can someone out there tell me how to stop it.

It might be a muscle spasm, you might be laying in a weird position or a position you aren't used of.

I'm flat on my back with my arms by my side parted from my torso with my legs apart.

It's called a hypnic jerk. I experience them and they come in a wide variety of involuntary movements and sensations. I believe the underlying cause has to do with the nervous system. So the first thing you want to consider is how medication, supplements, recreational substances, etc might be affecting your body. Beyond that, I've observed two things for me. First, it helps if I keep my elbows and knees slightly bent. If they are locked, then the stiffness and discomfort will eventually cause a spasm. Second, the jerks seem to happen when I'm trying to WILD when I'm not sleepy enough. So when I start, I spend about 2-3 minutes relaxing and then I test my state? How sleepy am I, really? If I'm very sleepy, I abort the WILD and just go to sleep. If I'm not sleepy enough, I get up and out of bed for a while until I'm more sleepy. Only when I'm in the sweet spot of sleepiness do I continue with the WILD. Then, even if I experience a jerk, I just take a few deep breaths and start over. It's a feel you develop with practice.

I agree that it sounds like a hypnic jerk. But I think, that it's a normal occurence. I'm pretty sure it will not happen next time. Unless you get them also when falling asleep normally, as in not WILDing.

Thanks for the replies guys, I'll try the method Sisyphus gave me and I'll get back to you's.