The relationship between light and dark (day and night) is what we call the photo period.
Flowering starts when the days get shorter. The growing and flowering of plants influenced
by a hormone (phytochrome). This hormone is produced day and night by the plant, but is
broken down during the day by light.
The hormone prevails as soon as the plant is in the dark 12 hours a day, and ensures that the plant will flower. The least amount of light during this dark period can cause it to bloom become seriously disturbed or even stops completely. That is why it is important to never enter a dark room, not even with a flashlight.
If you turn the light on at 18 o'clock then this hormone will never get the upper hand and remains the always plant at the vegative stage and therefore grow.
Light is also important for generating energy.
6H2O + 6CO2 -- Light à C6H12O6 + 6O2
Water + Carbon - Light à energy The more light, the more energy the plant can make if there is enough water and CO2.
A trick that some growers do is to light up in the last 2 weeks of the flowering period from 12 a.m. to 2 p.m. The plant then has enough flowering hormones in it to keep on flowering but get 2 hours a day longer light (and therefore more lumen) to take on energy (and 2 hours less dark to use energy).