You know you're burned out when things that used to motivate you to get up early and work through lunch now feel like burdens. Not just work stuff either, but burnout it starts affecting your life outside work, on things that you thought had nothing to do with work.

> a challenging situation

You're challenged when, even if you don't know how to get through it, you have in your mind a structure, a way of getting through it. You know you can figure it out, you just have to work it out, and the process of working through it energizes you. You're over-stressed when you don't see a way through, or when you see the way through but dread doing the necessary things.

Tolerance for stress depends, in part, on a person's level of resilience: your ability to face reality exactly as it is, your sense that what you do will have some meaningful impact, and your self-confidence in your ability to handle the situation. You might remember a time when you were a kid and something you dealt with seemed like the end of the world, but now you look back and wonder what the big deal was. Your ability to manage stressful situations grew.

In short, when you're burnt out, you're tired even when you get enough sleep, you are reflexively cynical and critical of the work, and you begin to feel like problems you should be able to handle require extraordinary efforts. Those feelings happen now and then when you're not burnt out, but if it becomes chronic, that's burnout.