I would also have said not enough light like sparrow but i notice u have foil lining along the wall of the plant room ? What light set up r u using ? How long do you leave your lights on for ? Sometimes the plants can become so tired if the light is left on for periods to long, also if your using foil and a lamp then the heat may be causing the leaves to curl under like that. To be honest the plant doesnt look that bad, not a brilliant pic but the leaves aernt shrivelled just curling under a bit. I wouldn't worry bout it unless they get dramatically worse. Try what sparrow said, if that don';t work might be summing iv'e mentioned.

Good Luck Dude