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.