There is a running joke among DC Comics fans that Superman is only ever as strong as the story needs him to be. One story might see the Man of Steel hold a black hole in the palm of his hand and sneeze galaxies out of existence, while another would see him struggle to overpower an ordinary mook with kryptonite bullets. The writers at DC Comics are well aware of this strange fluctuation in Superman's power levels and attempt to provide a reasonable explanation in Action Comics #1030.
In the comic, Superman believes he is dying of an unknown medical condition. His friends and scientists The Batman and Atom run tests, and confirm that the Man of Steel's body is experiencing some form of cellular decay. Batman believes this could be some form of radiation poisoning. But Atom has a different theory, which ties into Superman's powers, which are a result of a lifetime of soaking in the rays of Earth's yellow Sun.
As Atom explains it, the Sun produces its heat through continuing solar cycles, which last roughly eleven years in each cycle. In that cycle, the Sun's heat output varies from time to tie, as the surface of the sun builds energy to the point where the magnetic field of the Sun flips, and the cycle starts again. Since this means that varying amounts of energy reach Earth from the Sun, this would also mean that the amount of energy available to Superman varies over time, consequently making him more or less powerful, depending on the solar cycle.
For his part, Batman is unimpressed by Atom's theory, but you have to remember the latter is a celebrated physicist, and may actually know more about this matter than the Dark Knight himself. Clearly, the writers don't want to write themselves into a corner by insisting this theory is the definitive explanation for Superman's fluctuating power levels, but are throwing the explanation into the mix for future storylines.
This is not the first time the comics have explored Superman's complex relationship with the Sun, and what would happen if that relationship was to change ever so slightly. In the New 52 run, Superman experiences "Super-Flares", when the collected energy inside his body explodes in a cataclysmic blast that destroys everything around him but also temporarily depowers him.
Meanwhile, some stories set in the future tell the tale of Superman Prime One Million. This is a version of the Man of Steel that has lived in the heart of the sun for thousands of years. As a result, his body has taken on a permanent gold hue, and enhanced Superman's powers to an impossible degree, making him a literal god.
The live-action movies are unlikely to do a deep dive into the logistics of Superman's abilities anytime soon since Warner Bros. is still struggling to crank out a Superman franchise that appeals to the general, non-comic-book-fan masses. Hopefully, the upcoming Superman movie reboot being developed by J.J. Abrams will bring the character's cinematic legacy back on track. This news originated at Screen Rant based on recent DC Comics.