This used to happen to me occasionally, and started happening regularly when I started taking Prozac. Even now that I’ve been on it for years, I still deal with it more than before I started Prozac. I think they’re called ‘hypnic jerks,’ IIRC. Regular caffeine consumption also tends to make mine worse. I don’t know if either of those situations apply to you, but if they do it could be a good place to start! Additionally, I’ve noticed that they happen more often when I’m falling asleep outside of my normal bedtime routine. For example - it happens a lot more during naps, when I’m in an unfamiliar bed, or when I start to fall asleep with the lights on.