I believe it is supposed to open at 4:30am, coming from the check in desk area. No idea about domestic to US airside at that time of the day. OP, I would find an empty gate and sleep as you get off the plane (assuming you do not have bags to collect) then go through US pre clearance once it opens. It's a while since I was in T1 domestic, but I see people camped out in T3 when I arrive for early morning flights down to Mexico. A lightweight air mattress may make the proposition a little more comfortable.