system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we twitch in our sleep?

I've noticed my boyfriend twitches when he first falls asleep. It seems to only happen when he's in light sleep. What causes muscle spasms and why do some people have it more commonly than others?
gpt
This is called a "hypnic jerk" and is completely normal. There doesn't seem to be a single cause narrowed down as of yet, but the prevailing consensus is as follows: 
 
At the onset of sleep, your muscles enter a state of restfulness. The brain, not yet fully in a state of sleep, interprets this stimulus as if your body has lost its balance (i.e. the sense that you're falling) and responds by recruiting a muscular reaction to regain your balance.