Leaves perk up in morning but hang down in evening.

Your plants are cycling for there rest period. Growth is like sprints for the plants. On close picture time delay videos you can see really well.

Also if your giving them a night period they drop just before lights out. Plants will learn a light schedule and adjust to it. What your seeing in that case is them getting ready for its rest at night

Plants can only intake so much light then they rest with lights on or off. Cannabis plants are C3 plants meaning they don't need a night period unless your trying to flower a photoperiod. I run my lights 24 hours and only use a dark room to flower photoperiods. Don't let droopy leaves bother you unless its been that way for nearly a whole day. This is my personal opinion and practice. The C3 information is public information put out by Cannabis studies