Light time

Optimally, you keep it the same... if you need it to change, don't be afraid to do it. re: circadian rhythms and whatever else. If in vege or autoflowers, i'ld leave them on until new schedule turns them off. Provide extra light, and if near max DLI with your current schedule, dim or rais the lights a tad for this 24 hour period, since you'll be providing more energy than normal in that itme. if in bloom, i'd go the opposite way so as not to mess up hormonal imbalnce.. ensure ~12 hours uninterrupted darkness or more (never less) and it'll turn out fine. *future ref, since you are clearly in vege or autoflower with 18 hours/day.