Regeneration cycle (The Christmas Invasion)

The regeneration cycle (TV: The Christmas Invasion, The Impossible Astronaut) was the time during which the process of regeneration occurred. Every time a Time Lord regenerated into a new incarnation, it would take fifteen hours for the process to be finished. During this time there were moments of disorientation as the Time Lord healed.

The newly regenerated Tenth Doctor credited the manifestation of a new hand to replace the one severed by Fadros Pallujikaa to having still been within the first fifteen hours of his regeneration cycle, which provided him with "just enough" residual cellular energy to regrow his severed limb. (TV: The Christmas Invasion)

Shortly following his regeneration, the Eleventh Doctor noted to Amelia Pond that he was "still cooking". (TV: The Eleventh Hour)

As they saw what appeared to be the Eleventh Doctor shot down whilst in the process of regenerating by an astronaut, River Song claimed to Amy that the Doctor had been killed in the middle of his regeneration cycle: "His body was already dead. He didn't make it to the next one." (TV: The Impossible Astronaut)