The twins are four years old when they curl up together in the same cot for the last time, their parents having bought them twin beds for their fifth birthday that arrive in the morning, ready to be assembled and slotted into two neat little corners of their shared room - Dipper taking the left wall as Mabel had already claimed the right with her pink crayons. They are both twitchy and restless little children by day, but they come from a family of deep sleepers that are not easily disturbed. However, as the years slowly roll on the more permanent habits develop and their sleeping behaviors change with their bodies.
At seven years old Mabel climbs into Dipper's bed to chase away the remainders of a scary dream that woke her up with a start in the middle of the night. Soon it becomes a regular habit, as the twins both find that sharing a bed seems to dispel the creepy shadows and glowing eyes that blink in the dark. However, their newfound differences cause this luxury to come at a terrible price – Dipper always sleeps completely flat on his back, whereas Mabel prefers to cuddle up with people, thus creating arguments with grumpy, badly muffled voices at half past two in the morning.
By the time the twins reach double-digit ages, they can barely stand to be in the same room; Mabel snores, Dipper drools, they both scratch and kick and mutter under their breath to the point where even their parents are disturbed in the night. They consider moving them into different rooms, but their small house has not provided the space adequate enough to do such a thing. They simply begin looking for newer, bigger houses in their state, and make plans to move by the end of the year.
When they are twelve years old they're back to sharing a room again at their great uncle Stan's, but Gravity Falls tends to change certain things in people's minds and thus the twins suddenly find themselves unsatisfied with their segregation back at home. This doesn't last long, however, because when they hit puberty Mabel frequently gets these painful cramps and doesn't allow anyone within five feet of her. She lays on her bed for hours at a time and gets irritated at her brother over everything, making it very difficult for them to share a room with one another.
Then just like that they're fifteen years old and they possessively guard their separate bedrooms, especially when their new and completely separate groups of friends come over for a visit. They still make the occasional venture across the hallway to chat with one another, but when they're staying in Gravity Falls they're worst enemies when the evening comes. Dipper wants to stay up reading with the brightest light in existence and Mabel wants to text her friends with the sound on so the little buttons go beep-beep-beep every single time she presses her finger to a key. These nuances among other things are what force them to hate each other no matter how close they are.
.
The twins are eighteen years old when they lay in each others arms for the first and last time in an eternity, both of them clinging like they'd never let each other go. It's the last day before Mabel leaves for college and Dipper moves to Gravity Falls – Grunkle Stan's age, it seems, has finally begun to catch up to him, and Mabel's passion for the arts has now become a vision. Their new lives are full of promises and excitement, but the fact that they will no longer have their other halves with them is enough to make them want to stay exactly as they are – living with their parents, going to high school, sleeping in the same room – anything just to hang on to each other. As they lay in the same bed with their arms and legs entangled, the last thoughts that linger before sleep are their silent requests. They've already promised one another to write and call as often as they can, but their true parting wishes are to hang on to their bond and never let their sibling love fade away.
Mabel still snores, Dipper still drools, both of them still kick each other and talk in their sleep straight through the whole night, but neither one of them pulls away.