If the light is made unavailable then the biosynthetic phase of photosynthesis

Immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops. This is because ATP and NADPH produced during the light reaction are used to drive the processes leading to the synthesis of food, more accurately, sugars. So when light is unavailable the light reaction stops and hence the production of ATP and NADPH also stops and ultimately when all the available ATP and NADPH are utilised the biosynthetic phase also stops.