Yes, cabin fever is real because staying indoors for extended periods of time has been proven to cause a negative change in the mood of the person. Some people stuck inside find themselves eating more and gaining weight. Being heavier than a previous weight tends to make us feel sluggish and results in a downward trend.
Also, we do need to be exposed to sunlight to create Vitamin D and to maintain our internal clocks that tell us when to get tired and sleepy and when to remain awake. It is also safe to say that for most people who are stuck inside, they don't get much exercise which a lack of also creates havoc for our bodies and minds.
For our bodies a lack of exercise can have you feeling restless and tired at the same time so you don't know whether you should go for a walk or lie down. And a lack of exercise can make your mind go into decline as well because our brains can improve and grow when we exercise. So, with all of these factors in play, it is easy to see that cabin fever is real.