Ambient complexity

Ambient complexity was an environmental condition which could cause a regeneration to fail. The solution to the problem of a "busy background" was, according to the TARDIS Index File, a Zero Room or an "eminently more effective" real, neutral location like the "habitation" of Castrovalva.

However, because the Master was manipulating the Doctor's TARDIS at the time Tegan and Nyssa read this information in the Index File, it was possibly inaccurate information. (DW: Castrovalva)