CAaI've noticed Autos seem to need a rest after 14/15 hours but as they go into flower they can take 18 hours and once you have buds say half way through you can 24 hour it without issue except more feedings, I don't know whether that might be the the R cannabis, I forgot the name, but could do with being at the latitude they grow in, it's possible that it doesn't go properly dark all summer.
I'm just guessing and I haven't read anything on the subject I've just been ruminating the past few weeks, that why now my F1 has actual big buds on I can leave it on 24 hour without it getting tired.
I believe they all need a rest though and will give it 18/6