Having a special type of eye structure helps nocturnal animal a lot.
The large pupils open wider & allows the maximum amount of light to enter the eye in dark. Their lens is large & situated near retina. This also allows a lot of light to register on the retina.
Their Retina contains 2 types at light sensing cells rods & cones.The Cones are responsible for colored vision & require bright, focused light.
On the other hand Rods are extremely sensitive to light
Rods have a photosensitive pigment called rhodopsin which plays vital role in night vision. Multiple rods are wired to a single brain connector. This helps in creating a stronger signal.
Many nocturnal animals have a mirror like layer behind Retina called " Tapitum lucidum". The light passes through retina & is reflected by tapetum ,that gives the retina a second chance to sense it.