It is a good idea not to water during the dark cycle. This is so the roots have a chance to aerate and also so that they will be good and hungry when the light does come on in the morning.

In my experiences it is perfectly normal for the plants to droop at night. They are literally resting. This is so the roots have a chance to grow a little more and so the plants have more energy for the next day. It could also be due to your temps dropping but if anything higher temps will make them droop rather than lower. If they perk back up in the morning then I'd say you have nothing to worry about. Just go get stoned and start thinking of the next thing to worry about. =)