Imagine you are sitting in your car at a stoplight. The light turns green, you put your foot on the gas, but the car moves forward very slowly. What's your default response?
You put your foot down more, right?
This is the same response your brain has when you're under stress and aren't overcoming it. Chronic stress causes your brain to change its physical structure to increase its stress-producing abilities.
Again, this may seem completely counterintuitive. However, as far as your brain is concerned, when you think about the job you hate, the relationship which has gone wrong, the expectations you can't meet, it believes you're in danger.
These micro-changes add up, and before you know it, your brain releases so much stress hormone and so little of the relaxation hormones you're constantly on edge. You can't think clearly, you can't sleep, you can't even digest your food properly because your body is in panic mode.
And remember, your brain IS NOT designed to make you happy. It is designed to keep you alive. There's no time to sleep when your life is on the line.