Why do my plants leafs drop half way through evey day?!

Hey so I have some heady plants that are a lil over 3 weeks old. I got em under a 400W MH with a 18/6 cycle, with about 80 degrees throughout most of the day.

They seem very happy early in the morning when I go take a leak at like 4-5 AM when the light fist turns on. The leaves are preky, prayin to the light. But then around noon, the leaves just slowly begin to drop down and by the time 8pm rolls around they really dont seem too happy?

Looks like a pretty serious problem. Any ideas whats goin on?!
 
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