Hey friend,
Technically you could drop it to 12 and then leave your tent open and then turn on the house lights for the remaining 6 hours. This will at least prevent them from being in complete darkness for 12 hours, they'll still be seeing some form of light and shouldn't get overly stressed as a result of the sudden and drastic light changes.
I know autos are a little more tolerant that photoperiods but still worth trying to keep some consistency. 👍