The Importance of Darkness

And why I don't recommend the 24/0 Light's on schedule...

Even if you're growing indoors, with a Hydroponic and HID light system, it's still important to practice the basic rules of nature. Where there is a Yin there is a Yang, and that is specifically true in nature. Plants need certain periods of darkness just as they need certain periods of light depending on the season. In the wild, plants will grow according to the "lights on-light's off" cycle. So for example if it is spring/summer(longer days, shorter nights) then the plant will take notice and use this extra light to grow more foliage. When the plant begins to notice the change into autumn/winter(shorter days, longer nights) then it will begin to end it's vegetative growth and begin producing flowers or fruit.

So what do plant's do in the dark?
During the bloom state they develop the necessary hormones in order to produce flowers and vegetables. During the seedling/vegetative state they use the darkness to grow roots and longer stems. While plants are capable of growing roots while the lights are on, they prefer the cover of darkness, and most of their roots will grow during dark periods. Ask any grower, farmer, even scientist, and they will tell you that in order to grow a successful plant you must first start with a successful root system. So by allowing your plant extended periods of darkness you will extend your roots/stems which will produce lush green foliage. However, too much darkness can cause your plants to "stretch" (stems growing too fast in order to reach light) so balance is the key; Yin/Yang

Here you can see the development of the root system over time

So why wouldn't the 24/0 Light Schedule work?
It's not so much that it doesn't work, it's just that the motivation behind it is ironic. The reason why growers leave the lights on for 24 hours is so that they will promote more growth. However, without darkness, or extended root/stem growth, they are forcing the plant to grow more foliage. While it seems like a good idea you are actually stressing your plant, and leaving it weak at the base making it more vulnerable to disease/pest. It's like a human taking steroids and only working out his upper body; strong muscular definition above, and weak chicken legs below.

So in conclusion, it's better to go with the flow of mother nature than against her. I personally stick to the 16/8 Light cycle for Seedlings (more darkness for more roots), and the 18/6 Light cycle for the Vegetative state (to simulate the longer days of summer).