Could be heat, also if you have a fan blowing directly on them it will cause that leaf curl or low RH inside your grow room. Look very healthy otherwise. During flowering I don't worry too much about that if my lights are a little too close. The RH though, if it is too low will cause rapid transpiration and won't allow the plants to take up nutrients properly, same with having the fan blowing directly on the leaves. The plant will take up lots of water but not to much nutrients and you will loose some yield from that.