The room is too quiet without Yamamoto.
It's a stupid thing to think. It's a stupid thing to even consider, when Gokudera has spent every night since he and Yamamoto started sharing a room listening to the other boy's breathing instead of sleeping. He has made something of a routine of it, staying up well past the point when Yamamoto falls asleep even if the light is on, only relinquishing himself to the attempt at rest once Yamamoto has been breathing slow and heavy in unconsciousness for an hour or more. Then he seethes, lies wide-eyed and glaring at the ceiling while the regular rhythm of the other boy's inhales sweeps over him like waves, considers how frustrating the sound is and how much better he would be able to sleep if he were alone, until he's jolting awake to the sound of Yamamoto yawning in early-morning attempts at wakefulness.
It would be better if he were alone. He's thought about this night after night after night; all it would take is a silent room, without the proof of another's existence ringing nighttime loud in his ears, and surely he will be able to sleep without difficulty.
And it is quieter, since Yamamoto was carried off by his supposed trainer. When Gokudera reaches out to turn off the light now there's no half-waking murmur from the bunk below him, no shift of blankets as Yamamoto rolls over, no hummed "Gokudera?" if he moves too fast or makes too much noise. And in the dark there's no sound at all, not the rustle of sheets against each other or the whimper of dream reactions, and not that everpresent wave of breathing to keep him awake.
Gokudera doesn't know, then, why it is he can't sleep.
He's not mad, not nursing the low constant strain of irritation like he used to when Yamamoto was in the room. He doesn't feel much of anything, other than distantly warm from the blankets, his thoughts as perfectly blank as he could wish. There's nothing in his head, nothing burning sensation under his skin, nothing but echoing quiet in his ears. He can hear his heartbeat, if he listens, can hear the catch of his own breathing for the lack of anyone else's, but that's not keeping him awake, there's nothing at all to explain the calm, relaxed, persistent insomnia, worse than any he's ever had before.
It's not that he thinks the bottom bunk will be more comfortable. He's not planning on sleeping there, just taking advantage of the change of position to read, maybe, anything better than the futile attempts at sleep in his own bed. But the blankets are still rumpled, the sheets still show the indentation of Yamamoto from when he scrambled out of them the morning he left, and when Gokudera moves it's so reflexive it startles himself. His knees drop him forward, send him toppling into the tangle of the blankets, and they're cold with loneliness but the pillow still carries some familiarity, the soft of the fabric clinging to a whisper-faint hint of grass, the smell of rain on hard-packed soil. Gokudera reaches out without thinking, drags the pillow in against him, and not even lying on it as much as clinging to it like it can give him the reassurance he is shaking with want of.
It hits him at once: the sting of isolation, the worse that now he knows what the alternative can feel like, the heavy weight of half-frantic worry like lead in his stomach. The silence is oppressive, crushing him down against the sheets, and even when his breathing scratches high into the shape of sobs it's not enough to fill the waiting quiet. Gokudera doesn't fit into Yamamoto's bed - he's all wrong, all sharp elbows and bony hips instead of smooth lines of unconscious elegance - but it's still better than his own, even if it makes the absence worse by recognition.
Gokudera can't sleep, even when the tears subside and he's left hiccuping against the other boy's pillow. It's not enough to be a substitute, not enough to soothe away the oppressive weight of loneliness. But in Yamamoto's bed he can relax into the hope for something better, can believe, if only for the night, that he'll be able to sleep again eventually.
In Yamamoto's bed, Gokudera can believe Yamamoto will come back.