It's hard, but they manage.
The first night no one can sleep. They all collapse onto their beds and let their numb limbs relax, too tired to do anything else, but no one in the Pines family sleeps that night.
The next night is better. Dipper falls asleep at Stan's side in front of the TV, and Stan falls asleep too, his head resting on top of Dipper's, who is leaning onto his arm. Mabel follows soon after them. Even Ford eventually joins, as he spots them when on his mission to get a cup of coffee to take down to the lab, drapes a blanket over his resting family, and nods off in the chair next to them.
The following nights get better, as they develop a pattern. Dipper and Mabel will often sleep in the same bed if they don't stay with Stan and Ford, who will often wordlessly sit together in front of the TV and nod off, slightly more at ease for being in the same room.
Then Mabel and Dipper leave, and Stan and Ford can't sleep for a few nights. It gets better, and they find that they sleep better after having called Mabel and Dipper.
They talk to the younger twins as often as possible. They're doing better, if marginally. They sleep soundly as long as they're in the same room. They still have bad nights, though, where the nightmares are worse, where they die, where the other dies, where their Grunkles die, where Bill wins. On the bad nights they'll crawl into the others bed and the presence of the other will lull them to sleep.
School is hard, and math is impossible. The triangles seem to follow them, refusing to leave them alone, making it hard for Mabel to breathe, forcing Dipper to run to the bathroom to puke up his breakfast, breaking both their concentrations and giving them nightmares. Dipper wakes up screaming, and only after seeing his reflection with normal, non-yellow, non-catlike eyes does his heart stop thundering in his chest.
Most often it's Mabel who calls their Grunkles. Dipper will often say hi and ask about new mysteries, but it's Mabel who will sit for hours and hours and hours, talking to both her Grunkles. She'll ask about them, about their days, about the town, about Soos, about her friends, about anything she can think of, and she tells them everything. She tells them about breakfast, about the school lunch, about the stupid English assignment, about math, and the triangles, about the nightmares.
When her voice runs out and they have to go to bed, Mabel sleep soundly, which means Dipper sleeps more soundly than he would otherwise.
And with time, a lot of time, the nightmares fade away. They stop flinching every time they see a triangle (Mabel does, at least, and she doubts Dipper ever will), they stop waking up in the middle of the night.
By the time they go back to Gravity Falls the next summer, they're ready to move on to other adventures.
I warned you it was super short
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