The leaves are drooping to protect themselves from the light. The reason is that the plants are being grown in small containers and the roots cannot grow enough to support the top part of the plant. A few hours before darkness they are pooped and use the dark period to recover.
The solution is to re-pot the plants into larger containers. The roots will quickly colonize the new medium and will be able to support the plants now that it has a larger infrastructure. Plants can flourish under continuous light during the vegetative cycle so long as the temperature is in the desirable range, and their nutrient needs are met.
Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA. You can also email Ed at AskEd@cannabisculture.com, and send queries via his websites at www.ask-ed.net. All featured questions will be rewarded with a copy of Ed's book, The Big Book of Buds. Sorry, Ed cannot send personal replies to your questions.