Regeneration sickness effects each incarnation of the Doctor differently, from becoming disoriented, partial amnesia or even acquiring uncontrollable anger. However, when the Fourth Doctor fell from a tower and regenerated into the Fifth, he was faced with what many fans believe is one of the worst cases of regeneration sickness ever seen in Doctor Who. This version was entirely unstable and was phsyically and mentally breaking down from the stresses of the regeneration, possibly due to the regeneration happening so far away from the TARDIS, which is known to help in some way with the transition. He alerted his companions, Tegan and Nyssa, that he needed to be taken to the Zero Room, free from the chaotic nature of the universe. Once inside, he was able to quickly revert to the Doctor of old who was able to peacefully rid himself of the regeneration sickness... until the room happened to be jettisoned by mistake to escape the Big Bang. Lesson Learned: Everyone should have their own "Zero Room", free from outside influences, where they can reflect upon their own thoughts with no distractions.