Calling them light and dark reactions is actually kind of misleading because both processes are actually occurring simultaneously and all the time in the plant.

For that reason, we’ll call them light-dependent and light-independent reactions. One of the coolest things about this process is that plants have learned to allocate their energy reserves to sustain metabolism and growth throughout the night. This means the simple answer to your question is the NADPH and ATP are never used up, and the Calvin cycle never grinds to a halt.

During the day, the plant uses all the light it can get to drive the formation of ATP and NADPH and the creation of starch. At night, the plants break down that starch to fuel continued growth.

This is similar to how we eat all day, build up an energy reserve of glucose, and then use those stores to fuel our bodies while we’re asleep. A recent study found that if we take plants and shut off the lights early (8 hour day, 16 hour night), the plants will actually adjust their normal nightly rhythm and metabolism so their energy will last until sunrise.

This depends on the plant. Plants that use crassulacean acid metabolism (CAM plants) do most of their fixation at night, but most plants do it during the day because it's more efficient not to have to store up your NADPH (Nicotinamide adenine dinucleotide phosphate) and ATP and wait until after dark to fix carbon. CAM plants do this generally because fixing carbon costs water, and by doing it at night when the temperature is lower means that less water is lost. As a result CAM plants typically live in deserts, salt flats, and other locations where water is scarce. Normal plants do their fixation in the daytime and so can grow faster and out-compete the CAM plants, but only if they get enough water that they don't dehydrate.