There are actually some physiological processes that take place only during the dark, making a brief period of darkness at the very least better than continuous lighting.
Oldtimer did an experiment where he compared different photoperiods and the best yields per kWh electricity usage came from 20/4 lighting but that extra couple of hours of 18h light a day does add up over the long run. So you have to balance electricity costs with yield expectations and for me 18/6 is the most practical. If you are using a 400W light it would be far better to upgrade to a 600W light than to have more than 18h light per day because they are so much more efficient, so it's a much more prudent choice.