I've just realised i've missed my dose of hydrocortisone today (15mg) it's midnight now - do i wait until the morning now to take my dose?

ASAP: If you are taking it for your orthostatic tachycardia, take it ASAP and resume the regular schedule with the next dose even though it may be just a few hours apart. Talk to your doctor if you experience any problem.

Take it now,and: resume your regular regemin tomorrow.Set an alarm , if necessary,so as not to miss another dose.It can be dangerous to miss.