Depression can have a lot of different symptoms:
a low mood, loss of interest in things you’d normally enjoy, changes in appetite, feeling worthless or excessively guilty, sleeping either too much or too little, poor concentration, restlessness or slowness, loss of energy, or recurrent thoughts of suicide.
If you have at least five of those symptoms, according to psychiatric guidelines, you qualify for a diagnosis of depression.
And it’s not just behavioral symptoms.
Depression has physical manifestations inside the brain.
First of all, there are changes that could be seen with the naked eye and X-ray vision.
These include smaller frontal lobes and hippocampal volumes.
On a more microscale, depression is associated with a few things:
the abnormal transmission or depletion of certain neurotransmitters, especially serotonin, norepinephrine, and dopamine, blunted circadian rhythms, or specific changes in the REM and slow-wave parts of your sleep cycle, and hormone abnormalities, such as high cortisol and deregulation of thyroid hormones.
But neuroscientists still don’t have a complete picture of what causes depression.
It seems to have to do with a complex interaction between genes and environment, but we don’t have a diagnostic tool that can accurately predict where or when it will show up.
And because depression symptoms are intangible, it’s hard to know who might look fine but is actually struggling.