In order for the ship to function properly, heat must be regulated and controlled. The heat and light from the Sun can cause any part of a spacecraft facing the Sun to superheat, while parts in shadow can fall to extremely low temperatures. This can cause problems for spacecraft systems. In addition, the spacecraft's own systems can create heat, which must be kept from building up and harming the spacecraft. NASA engineers try to maintain a balance between heat sources and heat sinks on the spacecraft so that the amount of heat generated or received equals the amount used or released into space.
More about temperature regulation:
How and why do we control heat on a spacecraft?
How is DS1's heat balance maintained?
What is heat balance?