You can often tell when you're coming down with something. The runny nose is easy enough to deal with, but then you also get congestion, then you start developing a cough. And at some point, you know you need to take some time to rest and heal. So, you take a sick day and watch some Netflix on the couch and then you are able to get back to full speed the next day.
The same process also applies to your mental health. If you can feel yourself declining, it's a good idea to take a mental health day to rest and heal. Taking a day to focus on yourself is not selfish. It's crucial that you take care of yourself both physically and mentally so that you are able to tackle the challenges that may be going on in your life.