As Cary Mitchell, a horticulturist at Purdue University told NPR, in the 1990s, “research showed that you could grow lettuce in just red light. If you add a little bit of blue, it grows better.”
I dont know about need, but a lot of weed farmers feel that plants do better with a rest/dark period. I haven't noticed a difference but give them a rest period anyway as its "natural"
I tried growing a plant in a closet once. The instructions I was given were to leave the light on 24/7 until it was as big as I wanted, then to switch to a 12 on/ 12 off cycle to get it to bud.
I didn’t try very hard on it and ended up with a quarter of mid weed after like 6 months