The movement on Jack's end startles Sinclair at first, but it's nothing really. Although he does recall the last time Jack slept...that didn't turn out too well. With the way Jack stirs, it occurs to Sinclair that maybe he's having another nightmare. He weighs the pros and cons of waking him up. Maybe it would be a kind thing to do, bring him back from wherever he is. But then again, he needs the sleep. He's got to.

Plus, if it's a nightmare and Sinclair wakes him up, what if he starts crying again? He doesn't really know what to do with tears, and he doesn't really want the practice.

He opts to leave him alone. If it is a nightmare, he'll be alright. And if it's not, bonus points for not disturbing him.