It had started from a very long time ago, ever since any of them could remember. They were first aware of it when Mikey knocked Raph off their shared bed, ignited a pillow fight between the tots in the middle of the night. From that day on, with the help from master Splinter, they came to know about their own movement during bedtime.
Mikey's was the first to be discovered, since it's quite obvious. He had the tendency to thrash around, not less than once they found him sleepwalking around and on them, waking them up almost every night just to fix his sleeping posture or guide him back to bed. It wasn't until later that master Splinter revealed to them about the youngest's hyper activities during the day was the cause of such habit. Even so, Mikey could hardly be limited even by their father, and his habit lived on.
Raph's was next in line. In their few fortunate nights that Mikey didn't thrash around so much, they were often disturbed by his snore. Sometimes, it could get loud enough to wake even the deep sleep Mikey. For this, they decided not to trouble Splinter about this, and thought of "fixing" the problem themselves. However, they soon found out that the task is more difficult than they anticipated. Every well known methods Don could dig out was proven to be useless against the emerald tot. They finally deemed it incurable and learned to live with it, along with Mikey's moving.
As if the two of their brothers weren't enough, Don's habit did, more or less, contribute in many of their sleepless nights. Master Splinter had decided to put Don in the teaching position, since his knowledge in science and other fields was much more advanced than the old rat. His brothers were very patient with him, for he often spoke in a manner that, as Mikey put it, "like a whole different language". But that's the least of their problem as the night came. Formulas, lessons, experiments... Information that they would never be able to comprehend was constantly recited, way after the genius had shut his eyes, and with clarity, no less. They could endure a soft slur and some mumbling, but not the olive tot's entire brain shoving to their ears. Just like with Raph, they all had to cope with it, whether they liked it or not.
Leo's was the last to be found, and in a very unhappy way. Master Splinter gathered them all together to inform about it, after countless night he found his oldest tot crawling to his bed. Not because Leo was wimpy, but because he couldn't get a decent sleep with Raph, Don and Mikey opening an orchestra on the bed right next to him. A very light sleeper, that's what he was. Even the faintest sound, the lightest stir could disrupt him, and their father's room was the only place with enough silent for him to spend the night. It was, by that point, the last drop for the wise rodent, leading him to the decision that they all need their own room, with their own space.
Years had passed, and without correction, no doubt their sleeping habits were kept until this day. But, an interesting development had occurred, and master Splinter found it to be, in a way, amusing. He was but an old rat now, with much less sleeping time required. He occasionally spent the night visiting his sons, checking them to see how they were in their sleep. More than once he saw that two, or if he was fortunate enough, four of his sons sharing the same bed. What surprised him wasn't the position his sons were in though, but the lack of sleeping habit that they displayed.
Wrapped in emerald green arms, the youngest didn't even move an inch. The only hint that he was a living creature was the constant rise and fall of his chest. His hothead son, right next to him, bury his head in the crook of the genius's neck, gently blowing out breath through his nostrils, but not a sound from his snore. The olive green one, flat on his shell, loosely let his mouth fell agape, but no word came out, not even a slur. Finally, his eldest son, rested his head on the arm of the genius, fast asleep like a baby. He should have woken up the moment the old rat opened the door, but he didn't. Splinter lightly chuckled before exiting, leaving his sons in their slumber. They later, much later, revealed to him that they have been... bonding in a way that he might not approve. The wise rodent could hardly suppress his amusement as he observed their shocked expression when he told them about his nightly patrol.
The only thing he needed to fix had been fixed, what more could he hope for?