The reactions of Calvin cycle not directly dependent on light, but they usually do not occur at night. Why?

Calvin cycle that take place in stroma of the chloroplasts during photosynthesis, is light independent .But they usually do not occur in night because this cycle is dependent on the products of the light reactions. The Calvin cycle proceeds in three stages: carboxylation [${\mathrm{CO}}_2$ is covalently linked to a carbon skeleton (RuBP)], reduction (carbohydrate is formed at the expense of ATP and NADPH) and regeneration (the ${\mathrm{CO}}_2$ acceptor RuBP reforms at the expense of ATP.