The symptoms of anxiety, irritability, and depression often are related. That is because they can different manifestations of the same underlying imbalance. It is not all in the head. The body and nutrition can affect these moods, as can various effects on neurotransmitter and gut flora balance. Some say they symptoms are related to inflammation of the brain.
Some books to give more insights are …
The Mood Cure starts off Chapter 1 by saying …
“If you’re often feeling depressed, anxious, or stressed, you’re not alone. We’re in a bad-mood epidemic, a hundred times more likely to have significant mood problems than people born a hundred years ago.”
Unstuck: Your Guide to the Seven-Stage Journey Out of Depression is by James S. Gordon M.D. who says that “Depression is not a disease”, but rather that …
“stress and the action of stress hormones are more likely causes of depression and of observed changes in neurotransmitter levels.”
The Chemistry of Calm: A Powerful, Drug-Free Plan to Quiet Your Fears and Overcome Your Anxiety talks more about the biochemistry of mood.