With the ramping we are doing with our lights these days, there are almost 2 photo periods.
1 is the complete time the lights are on. Typically 10-12 hours
2 is the period of maximum intensity. typically 6 hours.
For example, my 180 with custom LED. 1/2 lights on at noon and ramp up for 90 minutes to 90%. then at 3pm, the other 1/2 come on at 100% (non dimmable). these turn off at 9pm and the first 1/2 dim down until 10pm. So full intensity is 6 hours. 1/2 Intensity or less is 4 hours.
on my nano, it is 6 hours of full intensity and 8 hours of partial.