Changing the light cycle 3 weeks into flower

My light cycle is currently 8am to 8pm and i need to switch it to be on at night and off during the day because the days are getting too hot.

To switch i need to have one of the periods go for 24 hours should that be the day or night period? Im guessing night?

You be guessing right! The long night will reinforce flowering were long day would slow or stop flowering.