<p>Light conditions have substantial effects on living organisms through a variety of aspects of individual life, such as survival, feeding, reproduction, stress responses, and behavioral rhythms. In this study, to evaluate the importance of light conditions for nocturnal fish, the spontaneous spatial preferences and swimming activity of the loach (<i>Misgurnus anguillicaudatus</i>), a nocturnal freshwater fish, was investigated using controlled light intensity gradients. Despite observing individual variations in dark preference responses, these preferences were not consistent at the individual level when measured repeatedly at four-week intervals. This suggests that individual differences in light/dark preferences may reflect state-dependent variations rather than fixed traits. When the light intensity in the bright area was changed from 2.4 to 400 µW/cm², the loaches showed a non-linear relationship between light intensity and light/dark preference. These results suggest that loaches usually avoid dim areas near the light–dark boundary and sometimes change their light/dark preferences. This behavior may contribute to their compartmentalization of safe dark areas surrounded by barriers and their habitat expansion by occasionally climbing over the barriers and migrating to suitable locations further away. Overall, this study compared the light-intensity-dependent behavioral patterns of nocturnal loaches with those of diurnal animals and proposed a model to explain their photoregulation.</p>