As each Doctor’s time comes to an end, that is when the new generation is about to happen. Each Doctor deals with their regeneration slightly different. The process of regeneration biologically is mostly the same, a shimmering light of time-vortex surrounds the doctor and shoots out of his face and hands. However, the lead-up and post regeneration are completely different. Let us also not forget that evolving technology has been a factor in the showing of regeneration.
Let’s start with the first Doctor’s regeneration. When the first Doctor regenerates it simply shows a light that nearly goes completely white across the screen, and then there lay the second Doctor. Not very eventful, but they did what they could for the time period.
From now on, I’ll be describing the New Who Doctors or this could turn into a very long post. Christopher Eccelston’s 9th Doctor regeneration was one of the more unfortunate one’s because he was nearly forced into it. After Rose absorbs the time vortex he understands he can no longer let her hold onto that kind of power. He absorbs the power from her and in doing that, it over loads his system and only has moments to regenerate while Rose watches. It takes about ten seconds for the change to occur, before the Tenth Doctor appears and asks “Where were we? Oh yes! Barcelona”.
The Tenth Doctor’s regeneration was also a sad one because it was a huge sacrifice. However, this David Tennant’s regeneration is drug out over about 10 minutes or so. After he absorbs radiation instead of Donna’s dad, he remains the same and says a final goodbye to all of the companions he has worked with, including Donna and her family, Mickey and Martha, Jack Harkness, the granddaughter of the woman he met as “John Smith”, and lastly Rose just before she met the 9th Doctor so he did not have to jump universes. Then, he finally returns to his TARDIS to complete his transformation but nearly destroys the time ship in the process. His regeneration takes approximately 8 or so seconds
Spoilers for the Eleventh and Twelfth Doctors ahead, read at your own risk!!!
The 11th Doctor’s regeneration was confusing. Because he is with Clara, imagining Amelia Pond being there, and then leans down and instantly is changed to Peter Capaldi. It was abrupt and I don’t think many Matt Smith fans got true closure.
Lastly, the 12th Doctor’s regeneration was similar to the 10th to 11th’s in physics of the TARDIS out of control and the door flying open. However, it is diverse from all the rest because after his transformation we learn the Doctor now has the capacity to be a woman.