Absolutely. Dark period is important, since the canopy cant grow without light, and the roots doesnt need light it gets to put all its energy into root production.
I have even heard some people say that any light during dark period can cause problems, even as little as ambient street lighting for outdoor gardens. I have never noticed that though, previous indoor crops of lettuce did fine with a little indirect light during dark time, this current aeroponic crop is the same way, light ambient lighting during dark time.
Now for my garden room with my HPS no light gets in at dark time and if I need to go into the room I use a green flashlight, since plants don't see green light. I am growing 16 peppers in single plant DWC buckets, tons of carrots in a flood and drain table(EBB&Flow style) and a banana tree in a waterfarm. They are thriving, well the banana tree I just got yesterday but the other plants are great! Only reason the lettuce isnt in there is cause it never did well with higher temps, its 84 in the room, with that much direct lighting and the heat they wilt in the aerochamber unless I run the pump constant, and then the water gets warm... Its an endless cycle of temperature nightmare. don't forget the even more important water temps I have to control in hydro/aero <70* at all times OR ELSE.