This episode is dedicated to Glenn Quinn, who played Doyle in the first season. Quinn died on December 3, 2002.
In Offspring (2001), the Nyazian prophecies foretell that Connor's creation will lead mankind 'into darkness'. Here it does exactly that.
The title is presumably intended to evoke the play Long Day's Journey into Night.
It is revealed that the Girl at Wolfram & Hart is actually a totem of the god Ra, one of the five Ra-Tet.
Gwen Raiden wants to escape to Tahiti. Angel creator Joss Whedon would later use Tahiti in Agents of Shield as a euphemism for the process that returned Agent Phil Coulson to life after Loki killed him in Whedon's movie The Avengers.