"Surely there have been times when you've been sad. Perhaps a loved one has abandoned you or a plan has gone horribly awry. Your face falls. Perhaps you cry. You feel worthless. You wonder whether it's worth going on," he wrote.

"Everything you think about seems bleak --- the things you've done, the things you hope to do, the people around you. You want to lie in bed and keep the lights off. Depressed mood is like that, only it doesn't come for any reason and it doesn't go for any either.

"At best, you tell yourself that your thinking is irrational, that it is simply a mood disorder, that you should get on with your life. But sometimes that is worse. You feel as if streaks of pain are running through your head, you thrash your body, you search for some escape but find none. And this is one of the more moderate forms," he wrote.