Ok without knowing ppms (too high...too low?) I'll offer two thoughts. I would not want my plants in 6.4 ph for that long but it seems to me you would have seen the issue beginning before tonight if the problem was ph.......

I'm assumong you've done something about the temps and your plants....no more 102 degree days and nights?

Lastly.....plants sometimes DO lower their leaves at night when they "sleep" If you suddenly interrupted the darkness they may look like that simply because they are resting their leaves. If THAT is the case they should perk up pretty quickly with the light on for awhile( and by pretty quickly I mean within an hour or so). If they DONT.....I'd then guess ph or ppms.... check both right now and make sure they are normal.