Your main character can’t sleep. They can eat, they can talk, they have a job, they are like most other characters, except for one fact: they cannot sleep. Not even a little nap. Maybe they used to be able to sleep, but something has changed, and they can no longer close their eyes and dream.
They do not have fatal familial insomnia, they will not die from this inability to sleep, but it will affect the way they live. However how it will affect the way they live is up to you. Set a timer for 15 minutes and then start to explore this alternative reality where your main character cannot sleep.
The details of this story are up to you. Maybe your main character is upset by this development, perhaps it effects there marriage, or confuses them. Perhaps your main character is grateful for all this extra time, they take up a second job, a strange hobby, or any number of other options.
It is up to you where this story goes, but it is a lot of fun to explore a scenario such as this one with a new or established character. Happy writing!