Answer:
Your nightly flinches are called 'Sleep Starts.'
Step-by-step explanation:
Sleep Starts are sudden, muscle contractions you may experience as you are falling asleep.
The reason you're experiencing this could be because you could be drinking caffeine excessively, or it could be because you consume nicotine (any amount). Or perhaps it's both.
Another reason could be because you're exercising way too hard in the evening. Although, exercise can improve sleep quality, exercising too hard near your bedtime could make you more alert before bed.
Another reason could be sleep deprivation. Try to limit your caffeine consumption, or try LIGHT exercising in the evening to tire yourself out. And a final reason could be stress and anxiety. Which can lead to insomnia.
Researchers don't know the exact cause, of course. But those reasons I stated above could help you prevent further 'starts' at night.