In 2015, I had a burnout. I had panic attacks, I was depressed, exhausted, I had bowel problems and I couldn't get out of bed. This was a terrible time.
My burnout seemed to come out of nowhere, as I thought that I was busy, but doing okay. I didn't feel too stressed and just kept going. Until my body said "No more!".
A major trigger of my burnout was my job and daily commute. I worked in a stressful software architect role as a freelancer and had a commute of 3 hours every day. But those weren't the only things that caused my burnout.
In fact, burnout is not just a work-induced syndrome. It is caused by prolonged chronic stress.
Stress is a natural human response to danger and other threats, that induces the "fight-or-flight" response by getting your body into a state of alert and produces (nor)adrenaline, cortisol and other hormones. This physical stress mechanism is trigger by mental stress, that can be induced by work, a long commute, worrying, conflict and more. And the stress response is also activated by physical things, like a bad diet, under- and over exercise, toxins in food and personal care products, and lack of sleep.
Mental and physical stress adds up. And when this stress doesn't go away, it becomes chronic and can lead to burnout. This was what happened to me.