Lets explain why a dark period is important, that way people are not confused as to why they do or don't want to introduce one. During the day, a plant does a few things. She uses photosynthesis to convert CO2 and light into energy, she stores extra energy for use later, and she pulls necessary nutrients from the soil (or water in hydro). While it she doing all of this, she is also converting energy and nutrients into plant mass (i.e. vegging) throughout the day. During the night, your girls are using the stored energy from the daytime to grow. Its not that your plant "needs" a dark cycle for proper growth, its that she is using 100% of her time and energy on building plant mass, not splitting it between growing and photosynthesizing.