My two cents on this is this: You should be delegating. If there is no HoS on duty, as HoP you should be looking to hire someone to take the slot, not taking the job onto yourself as you have other duties and departments to attend to. If you're spending more time being an HoS instead of an HoP, then you might as well change your ID and accesses to only HoS accesses because you are neglecting or distracted from your other duties.
Aim for delegating, moving people around, seeing what is up and who best can handle it. There are more pieces on the board than just you.