The cause of not being able to sleep is having too much thoughts in your head. You spend the night thinking about things, usually profound.
Once you start WRITING those thoughts on paper or typing them, you’re releasing all of the worry they cause. That tension that builds up in your muscles just goes away, so you feel worn out at the end of the day.
You don’t end up writing just that one single thought by the way, you end up writing a trail. The newest gets created from the previous and it keeps on going.
Even speaking doesn’t have this effect. You’re generally waiting for a reaction from the other person, but on the paper you have no one but yourself. It’s essentially a huge monologue.
And that’s all you have in the world, just yourself. Others can help you but they’re not going to be with you every step of the way.
Get used to it, and do it easily with writing.
You’re also not the only writer. Essentially every blogger, book writer, and other text meant to be read (but not contested) was written by those that felt the need to say something. If you can’t get your voice out of your head, it’s going to keep bouncing against the inside of your skull.
Write one sentence. You don’t have to, but if you feel like going, don’t resist. It’s really hard to start again once you stop.