Wilting Leaves turning yellow!

I had a Sativa designed for outdoors UK growing outdoors for 4 months. It was 3 ft when i took it in and put it in a bud box grow tent under 400W lights.

Ever since, the leaves have been dropping terribly and some lower down have gone yellow.

When the lights go off (it's on 12/12) it perks up a lot.

Any advice on what's going on? Is it just the heat? I have a fan on them, doesn't help..

bump
!!!!!!!!!!!!!

im not expert...but too much heat? what temps when lights are on?

little more info stranger , was it in a pot outside or did you dig it up??? and yea temp. and your not drowning it are you???

how far is the light from the top of the plant? since its HPS, it should be a 2 feet or so, also if you should make sure you have enough o2 in your soil, outside the soil gets way more o2, so what you should do it add some food grade h202 (hydrogen peroxide) in some water and water the plant with that, seperate from nute feeding