The leaves on my plants curl downwards…. why and how do I fix it?

First the quick rules:

  1. Young leaves curling; the humidity in your grow room is probably too high; open the door!
  2. Old Leaves curling: something is probably wrong in the root zone. Often it could be a bit of root rot. You can minimize the risk of root rot by stopping the watering 2-4 hours before switching the light off. The stonewool should only be 50-60% wet over-night.
  3. Sometimes your general humidity is just fine, but your plants are so bushy that there is not a good air movement between the branches. Use fans. As an example, I can see that my cherry tomatoes are most happy in 15 mph "fan-winds".

More technical detailed explanation to curling leaves
If the youngest leaves are curled downwards, it more than likely indicates a calcium deficiency. If old leaves are curled; something is probably wrong in the root zone (which also reduces the calcium uptake). 

Calcium deficiency is the result of insufficient water movement through the plant. Remember, calcium only travels in the water stream of the plant, not in the nutrient stream. Therefore, calcium deficiency is usually (90% of the time) related to the climate in the growing area. High humidity will prevent calcium uptake even if there is sufficient Ca in the feed solution. Also, large day/night fluctuations in humidity will disrupt the Ca flow within the plant and lead to blossom-end rot (BER).

Another cause of BER or obvious Ca deficiency in the leaves is poor root development. This is a result of low substrate temperatures or the presence of a root pathogen such as Pythium or Phytophthora. Calcium is taken up by the area of the root immediately behind the root-tip. If the roots are not actively growing, new root tips will not be formed and consequently, Ca uptake will be reduced.