The reason why the Sharingnan is so draining is that you're not just pulling stamina into chakra in order to do one thing. You're not just converting it so that you can see what your opponent is doing -- if that were the extent of it, you would just have the Byukagan. You would just be dealing with the chakra channels, and your eyes would go white.
When the Sharingnan turns on, your eyes go red. The Sharingnan system is the Byukagan system bound and assimilated into the circulatory and muscular system; when you look into the eyes of Sasuke when he has Sharingnan, you are looking down along the veins of his body and into the chambers of his heart. The Sharingnan gift is that you take the Byukagan and link it to the muscles of the body; you take the gift of insight and add the gift of mimcry. Ever time Sasuke sees something done in the Sharingnan, every time he opens up his eyes to see something, he has to physically will himself to keep from repeating it.
When Sasuke was young, he had poor chakra control. They had had to cover up all the mirrors in the house when he was a baby -- the Sharingnan had manifested itself early in him, back even before he had learned how to speak, and there was one afternoon where he had wandered off into an unused room of the Uchiha compound.
They found him a few hours later, locked in front of dusty mirror, Sharingnan on in both eyes and locked onto the mirror-reflected image of Sasuke. Sasuke had gone to stand in front of the mirror to look at himself, and he had tried to imitate a basic jutsu he had seen some of the seven year olds practicing out in the courtyard. The Sharingnan had turned on; it saw the boy in the mirror. It mimicked him.
Sasuke had been performing the same taijutsu for three hours when they found him.
He was a year and a half old at that point; when they pulled him out from that mirror, Sasuke had refused to open his eyes for days and had kept his face pressed against his mother's side whenever he could.
Sasuke had known about Kakashi since before he was seven or eight. Sasuke would occaisionally see him coming in and out of the Hokage's office at about the same time he was supposed to report for his weekly interview with the old man, and there was also something inherently fascinating about it even though Kakashi was separated from Sasuke by uncountable degrees of family separation. Kakashi had the Sharignan. Kakashi had set the record for the chuunin exam. Kakashi was the legend that Sasuke was erasing at the Academy.
Sasuke hadn't actually known about the Sharingnan, though, until he casually brought Kakshi up to one of his teachers in an informal tutorial session that Sasuke was getting because he already knew everything that they were teaching in the classes, and Sasuke had wondered out loud just how Kakashi had managed to do it.
The teacher was old. Fifty, fifty-five. Retired from the active service.
"Kakashi's father used to bring him to the practice fields after Kakashi's mother died. He had a thing about never letting Kakashi out of his sight if he possibly could, so we used to play with him during our lunch breaks and jog over in the summeritme to make sure he wasn't getting to cold or keep the warming seals on him during the winter. Kakashi never cried as long as he could watch us, and he was playing with our tools and learning jutsus by the time he could crawl."
A pause. The old man tapped his pipe against the floor to loosen the tobacco inside, then said,
"Your grandfather wanted to kill him, you know. He wanted to keep the Sharingnan in the main family."
The story to Kakashi's past goes like this:
Kakashi's father had been a Konoha ninja. A great-granduncle of his had married a Uchiha, and there was a trace of the Uchiha blood running in his veins, which meant that Kakashi's father could theoretically be included in grand Uchiha celebrations, which they started to do once he started doing well as a chuunin. He met a third or fourth cousin there; he fell in love; he married her. He went on a mission and came back with presents from the village he had been on a year-long mission to, and she confessed to him, when he got back, that the baby inside the house wasn't his, but rather, that of her cousin who she had been in love with for years. Her family had let him marry her in order to get her out of the Uchiha compound and away from this cousin, and she had started sleeping with him a month after Kakshi's father left.
She had not seen him since Kakashi had been born, and Kakashi's father went outside to get a breath of air. The way that the old tutor had told it to Sasuke, it had been in the springtime and raining, about the time of cherry blossoms.
Kakashi was two months old.
When Kakashi's father went back inside to tell her that he still loved her, that it was all right, she had already slit her own stomach and died in the front room, next to the pile of gifts that he had brought back for her from the Wind Country and with Kakashi still in his cradle on the other side of the room.
"I helped Kakashi's father rip up and throw away the boards in that house. There was so much blood, but his father didn't want to leave the house, and when Kakashi became a chuunin, Kakashi's father was still living there. He came to the ceremony, gave Kakashi his old knife holster, drank a cup of wine with us, then hung himself later that afternoon over the same spot that his wife had killed herself. Kakashi was the one the one who found him.
"We don't speak of this," the tutor says to Sasuke at that point. "I am telling you only so that you know other people in this village have had a past as sad as yours and don't harbor any ridiculous stories of revenge -- Kakashi has as much reason to hate his family as you do, but for him it is forgotten. You should learn to do the same"
After learning that story, Sasuke has found it profoundly ironic that Kakashi enjoys reading a series of books about romantic love and titled Come Come Paradise.
When Sasuke is training with Kakashi, there is this one part where Kakashi tells him that he is not looking hard enough, that he is not seeing as much as he could, so then he pulls out this full length mirror that is as tall as he is and sets it in front of Sasuke. Sasuke has no idea where or how Kakashi has managed to get a mirror that big out in the middle of the fucking boulder-ridden wilderness, but Kakashi sets it up in front of Sasuke, standing just behind it to hold it steady on the uneven surface. "Now, look in the mirror and try to see the chakra gathering around your hand."
Sasuke has to repress a little bit of a shudder at the idea of doing jutsu's in front of the mirror -- he doesn't actually remember the episode with the mirror from when he was a year and a half old, but the muscle memory of it is still there, and Sasuke has been told about it, so he knows why he's shaking a little when he forces the chakra out of the channels and into the tip of his fingers. He looks at himself in the mirror, he looks at the expression on the face in the mirror, and then he looks over a little and sees Kakashi, one hand holding the mirror up, the other holding that novel of his.
It never occurs to Sasuke that he and Kakashi are mirror images, that the mimics are mimicking each other.
Things seen out of the Sharingnan eye are never forgotten, but they are never properly remembered, either. Events recorded through those spinning pupils go to the same place that muscle memory and dreams on the edge of dawn go; you can never consciously or clearly pull them out, but they are always there when you need them. Another thing that Sasuke finds incredibly and enormously ironic is the fact that if he had been looking at the world with Sharignan eyes when his parents had been killed, he probably wouldn't have a vivid memory of what that night had been like.
On his part, Kakashi goes through the year or so of Academy classes that he does attend with one eye closed because he can never learn any theories or terms when he's looking at the world with a Sharingnan eye he cannot turn his Sharignan eye into a normal one.
In that way, he's not the same as Sasuke. Kakashi cannot control his left eye; Kakashi's father committed suicide; Kakashi's mother cut her own stomach open; Itachi stole any chance he had for revenge against the cousin who was Kakashi's father. Kakashi has had far fewer choices. He did make the choice not to hate the Uchiha clan, not to want revenge, and he had made his choice long before Itachi killed his parents and family, but he has always had far fewer choices than Sasuke, who is pure-blooded Uchiha and was supposed to be the heir to the clan name and fortune.
Sasuke has the choice of using normal eyes or Sharingnan eyes.
One theory and set of terms that both Kakashi and Sasuke learned in the Academy is that Sharignan eyes are red because of blood. Looking at the world with Sharingnan eyes is like seeing it with the glass and fogged layer peeled off. Everything looks clearer; everything is burned into the memory.
Sasuke can have Sharingnan eyes or normal eyes. He can have his choice.
And in the end, what it boils down to is that Sasuke can either look with eyes of blood, or he can go blind.
These are the exact words that the old Academy tutor used on Sasuke. When he said this, on a rainy spring day with the cherries blossoming outside, the old man had been thinking of blind in terms of a blind eye toward the past, a more forgiving eye for the shortcomings of other people. Sasuke, though, takes it in an entirely different way. Blood for revenge, blindness for forgetfulness, and Uchiha eyes don't forget. Uchiha eyes mimic, and Uchiha eyes record. Uchiha eyes lock away into the deepest, darkest parts of the mind whatever they see, and Uchiha eyes used to be known as mirror eyes and the Uchiha gift as the mirror talent. Uchihas could mimic anybody. They could learn any jutsu; they could be the Hokage's image, they could be your mirror image if they so wanted. This is a battlefield skill that Kakashi has learned to perfection, and it cannot be denied, either, that in a way, up to a point, Kakashi and Sasuke are mirror images of each other.
When Sasuke finally follows Orichimaru out of the village for the sake of revenge, though, the distinction between the two of them has been made utterly clear.