It's true that he hasn't slept yet. He's tried and failed to get into a more normal day rhythm for humans, but it's still rare that he's still awake by the time the sun goes up. It seemed important to keep watch today even though he has no reason to think Tyki will come for Ianto, especially not seek him out in Spike's apartment. It's more for Ianto's sake.
That said, the light knock on the door still puts him on edge, muscles tense as he quietly moves over to open the door. There's visible relief when he recognizes Jack's voice and he cracks the door open, ushering him in.