You may have found that cat eyes can change day and night. However, do you know indoor lighting can affect a cat's health? Circadian rhythm In ancient times, before artificial lighting was invented, a cat was active around sunrise and sunrise, because their prey moved out in the meantime. The only lighting human could use at that time was candlelight which was so dim that can hardly affect a cat ' s circadian rhythm.  What is circadian rhythm? It is usually called a biological clock or body clock that regulates a cat ' s inner sleep-wake cycle and repeats every day. Circadian rhythm can be affected by lighting, as it is connected with the length of day and night. Cat has a built-in reflector in the back of their eyes called a tapetum, which allows a cat to see with only one-sixth of the light that humans need, but it also makes cats '  eyes very sensitive to bright light by changing pupils from time to time. How sensitive cats '  eyes are? According to   National Wi