The lack of sleep bugs him no end, but he won't let the exhaustion get to him. Every time his eyelids fall near-shut in an aching, painful way, his active brain alerts him and they prise open at lightning speed. He can't sleep, because he doesn't want her to leave. He doesn't want to wake up to a cold, empty bed in a cold, empty apartment and have to deny anything ever happened with a cold, empty heart. Because he knows it'll break him.
So, instead of committing to the much-needed sleep he so desperately doesn't desire, he clings to her like a moth to a flame. He clutches her entire, bare body to his, feeling that if he lets go she'll take his lifeforce with her, leaving him a wreck in her wake. Her skin feels warm beneath his arms, her pulse beating to a calm, inaudible rhythm that he can feel beneath his hands. Her breath is warm and steady against his chest, her head nestled in the crook of his neck and her face turned downwards. His legs are tangled with hers in a way that would send shivers down his spine, were he not so frightened. He focuses on all this in an in-vain attempt at a cool and collected manner, but loses his train of thought as she writhes a little in his arms. He unwittingly tightens his grip on her form, but she moans, still asleep, in protest. As she nestles down in an apparently more comfortable position against his chest, she murmurs his name in her sleep, and his arms relax almost immediately. She smiles against his body and kisses his torso, and he pulls her closer- more gently this time- and closes his eyes in a sheer sensation of euphoric bliss. So he sleeps, because if her actions say anything at all, he gets it. She's not going anywhere.