some people do t to keep down electric bills. If you a huge grower with 6 1000 watt HSP systems running that 6 hours can save you a few dollars here and there ya know what I mean. Other do it because its just what hey do. If your plants are doing good with 18/6 leave it alone if you want to experiment the 24/0. You can do that as well. Up to you really I know I havn't noticed any real big difference in growth with the 18/6. Which is what I use. experiement maybe 24 hours will work better for ya.