why are my tomato plant leaves curling

tomatoes like to be shaded from direct sun in the greenhouse, so i think they are suffering from too much sun.

During the day they make far to much food due to the warmth. Then at night the temperature dips and the plant has a problem absorbing all its made. You then get a start stop problem which makes the leaves curl.

Do as Steve says and shade your greenhouse or damp down the floor with water (not too much though, you dont want mould), open the door and vents to get a breeze inside.

You are then trying to have the temp more even with out the big peeks of hot or cold temp.