What you’re feeling is called a “sleep start” – or technically, “benign hypnic myoclonus”. This is an involuntary muscle twitch that usually occurs in the lightest stage of sleep – right as you’re falling asleep or waking up. They’re fairly common, and some people describe the feeling of falling.

Sleep starts are a normal phenomena, but may be more likely to occur if you’re stressed, over-tired, or in an uncomfortable position.