The question as to whether cats get depressed is related to lifestyle and how they’re treated and not simply to being kept indoors vs being allowed to roam outside.
Research has settled the fact that cats do have emotions, thanks to the existence of a region in their brain that is very similar to the human brain.
While many cats certainly can live safely and comfortably indoors only, some of them develop emotional problems, such as depression, but it’s probably due to lack of mental stimulation related to lack of attention indoors. As with people, there are wide differences. Some cats don’t mind being alone while others absolutely need and will thrive on companionship and certain activities. This companionship may be with their human, or with another cat or two…or more. Or “all of the above.”
Not everyone is equipped to handle a solitary life, including cats, who may need emotional outlets for normal feline behaviors. If they are unable to express these needs, they may become depressed and will demonstrate what we call problem behaviors, such as scratching everything in sight, urinating outside the litter box, hiding, excess shedding, over-grooming, and even sleeping beyond the normal 16 or so hours per day. They may overeat or refuse to eat. They may vomit more often.
In fact, my cats do not sleep that much. For one thing, which helps a lot, they have numerous housemates, who often become playmates. They have me, a very involved cat guardian. They need to play often and get some exercise. I provide toys, cat-friendly furniture they may climb, and interactive games. They are given a wide variety of foods to satisfy even the pickiest appetites, and novel items to chew on, such as “cat grass” and raw bones. Others may prefer one of the many cat chew toys available these days.
These activities are often regarded as more easily achieved if they can go outdoors regularly, or even at will. Many countries favor allowing cats to satisfy their desires with easy access to the great outdoors. U.S. cats are more likely to be kept indoors by cautious and concerned owners to prevent unhappy outcomes, such as accidents, predator attacks, mean human attacks, and potential disease from contact with strays who may be ill.
Indoors, we have many opportunities for activities such as outdoor viewing from window perches, boxes to hide in or play hide-n-seek with the others, chasing a laser pointer, and the frequent favorite, catnip, though not all cats relate to it.
If going outdoors is important for your cat’s emotional well-being, consider supervised walks on a leash. Using a harness is preferable to a collar, as most cats have little trouble shrugging out of them. Just condition them to its use while still inside the house!
Another option is to build an outdoor enclosure, called a “catio.” I’m working on one this summer.