The Doctor's regeneration was based on a bad acid trip
Internal memos from the BBC's original series, unearthed in 2010, describe the 'metaphysical change' of the Doctor's regeneration as 'an experience in which he relives some of the most unendurable moments of his long life, including the galactic war.' (No doubt the writers of the John Hurt arc leapt in joy to read that.) 'It is,' continue the memos, 'as if he had had the LSD drug and instead of experiencing the kicks, he has the hell and dank horror which can be its effect.'
