Leaves Curling!!! What does it mean?!

Been growing indoors w/ a hydroponic system. Plant will be 15 weeks old soon- and the leaves are beginning to curl down and around themselves- First time grower- so I have no idea if this is good or bad!?! Does this mean I am close to harvest? Is there too much nutrient? Has anyone else had this happen to them? Any advice would be greatly appreciated!!!

Everyone else has had this happen for many reasons.

Pic's would help alot but we need way more info.
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If I tried to guess it would be a stab in the dark.