21 hours of darkness would be ideal for more reasons than just that, too (like energy savings and flowering probably two days earlier). I've never heard that longer nights are less stressful, I think you'd be fine either way if it's just one. But 21 hours of darkness is probably the right play in any case.
Hey growmie, if you want to really want to know whatâs best for the plant overall for light length, look into DLI or Daily Light Integral. If youâre reaching the DLI for cannabis in 16 hours, extra light isnât helpful.
Cannabis is also one of the few plants who donât need a nightly dark period for growth, still learning the nuances of that.
But if youâve got a weaker light, you can definitely get advantages if you can pay the power bill.
My current plants have been on 24/7 lighting for three weeks themselves, and cannabis also doesnât start flowering till it gets less than 14 hours of light, so some people even mess with 13/11 or 11/13 light cycles for flower too.