[Because you're the nocturnal one with the weird sleep schedule to work around obviously not at all because Ianto is worried.
Even being worried he knows when not to push him. He doesn't think Spike will do anything stupid but he can understand him needing space. It certainly wouldn't be the first time that's been his reaction to a personal crisis. So his reply is casual with a healthy dose of perceptive mixed in.] Alright. Don't make yourself too scarce, alright?