I've been there. Depression may hide behind decreased activity, sleeplessness, loss of appetite, weight loss, constipation, decreased enjoyment, and lack of interest. Depression may come on suddenly or so slowly it is not recognized. It runs in some families and may be a biochemical disease of the brain. It tends to be recurrent.
Depression can ruin a person's life.
Untreated depression may land you in a nursing home unnecessarily or may make you unloved and lonely. You can die unnecessarily from a depression. Suicide rates in the general population are 12.4 per 100,000 people and in the 80 to 84 year olds, the rate is 26.5. Don't wait and see. It won't be brief, an untreated, depression will last at least six months.