While Dean loves having his own space, he can't fall asleep without hearing the soft sounds of sleep his little brother makes. So he always finds his way into Sam's bed every night. Like a toddler learning to sleep alone. Just can't help but seek out the warm embrace of Sammy.
I feel like he slept like shit during season 6, too. Sam wasn't sleeping, so Dean wasn't sleeping. Bet he tried sleeping to the sounds of Sam typing on his laptop or cleaning guns, sharpening knives. But it wasn't the same. Wasn't grounding like the image of Sam sleeping was.
And jumping back to season 8, I bet Sam always secretly waits for the rattle of the door handle, and the nudge to scooch over. I think Dean would leave before Sam wakes up in the morning. And maybe they never talk about the little arrangement that formed.
