When growing cannabis, does the dark in the light-dark cycle need to be absoultely pitch dark, or would dark ish be ok too?

I read somewhere that any light at all during its rest time will fck with the plant in undesirable ways, but I'm kinda skeptical about that.

What you got to tell me about this?

If I have them under lights 18 hours , and 6 hours in a dark-ish room, is that ok too?