If its only when the lights are out i wouldnt worry about it.

Drooping leaves can be a sign of over watering, under watering, and heat, although more often than not the leaves will point up in the case of heat stress.

My plants at the mo are on an 18/6 and i have now proved to myself that plants grow quicker on 18/6 than 24/0.

My plants also droop in the dark period, not alot, but a little, but as soon as the lights come on in the morning they're back to they're normal happy weedy selves.