SHORT ANSWER
Depression is recognised by a lasting set of signs: low mood and/or loss of interest most of the day, plus changes in sleep, appetite, energy, concentration or feelings of guilt — lasting weeks, not days. One bad day is not depression; thoughts of self-harm are a reason for urgent help.
Why is that?
Why it is the "set" that matters, not one symptom: anyone has days with low mood, fatigue or insomnia. In diagnosis we mean a combination of several signs that persist most of the day and nearly every day for two weeks or more. That is why at an appointment the doctor clarifies not "are you sad" but how you sleep, eat, think and move — the picture is built from details.
What goes into the typical picture: low mood or irritability; anhedonia (what used to bring joy no longer does); sleep disturbances (insomnia or, conversely, excessive sleep) and appetite; feelings of guilt, worthlessness or hopelessness; difficulty concentrating; reduced energy; thoughts of death. Not all signs need to be present — a stable combination of several is enough.
Why it matters to treat: depression does not "pass on its own" in most people — without help it tends to drag on and relapse, and each new episode makes the next more likely. At the same time it responds well to treatment: psychotherapy, medication or their combination return most people to normal life.
WHAT TO DO
- If the picture is familiar and lasts weeks — see a specialist; do not wait "until it gets really bad".
- Thoughts of self-harm are not weakness or manipulation: urgent help, conversation with a specialist right now.
- For close ones: watch the trend (sleep, food, contact) — you may notice changes before the person themselves.