From the John McCain Memorial Day weekend medical record document dump --
Also revealed: He has occasional momentary episodes of dizziness, when he gets up suddenly. McCain first told a doctor about them in 2000 — a visit that also uncovered the melanoma — and intense testing concluded they were harmless vertigo. He didn't report any episodes at his most recent exam.
So if he sits up in bed to take the fabled 3am phone call, he faints.