Perhaps as a consequence of his chronic inability to bring stillness to his mind, Kaiba valued his sleep when he could get it. Each night, without fail, he made a series of motions that displayed a clear intention to sleep. He took a shower and put on pajamas. He closed the heavy blackout curtains in the vain hope that perhaps, in absence of light, sleep would be easier. He was a creature of habit after all, but that was often the end of nighttime predictability. Once he was actually in the bed, an ordeal of internal monologues and tossed pillows inevitably began.
Work was something that never, never left his mind. He lived his work. The company he ran had his adopted name right in the title. He was his work, what he did was the truest extension of himself that existed. If asked, he would almost certainly say that he wouldn't have it any other way, probably with no small hint of a smug smile. Nobody ever asked, though. The fact that Kaiba was synonymous with work (even if his work happened to be games) was a fact taken for granted by practically everyone who knew him.
Truthfully, Kaiba only thought about work to avoid thinking about other things of a more personal nature, pensive inward reflection and remembrances of the past. During the day, those were much more easily suppressed, but lying alone in the middle of this king-sized island of fabric in a completely dark room didn't exactly leave him much else to think about. When he was lucky, he could remind himself that this, like so many things, was just another waiting game, that exhaustion would win out eventually, and that perhaps then he would find a dreamless repose.
He always hoped for dreamless sleep.
He still had those nightmares sometimes, a lasting gift from that Yugi kid after his first and worst defeat. Yugi had leveled against him in one night a punishment worse than anything Gozaburo, in his long period of paternal tyranny, had ever done. A blur was all he really remembered, a flurry of teeth and talons and steel and sounds, horrible gut-wrenching sounds, and being utterly helpless as he was torn apart and devoured and torn apart again. For whatever reason, the real scope of the pain of that night had been mercifully omitted from most of the memories, but truly, he would have given anything if only not to relive it ever again. He had died. Literally. Repeatedly. And sometimes, when he dreamed, he died again.
Despite the vital worth of a good rest, those nightmares were sometimes just not worth it.