Economies, stock exchanges and the housing market all get depressed. Weathermen tell us that there are depressions in the mid-Atlantic. We complain of depressing football and election results; and Monday mornings and TV repeats can be depressing too. With so much 'depression' around, it's unfortunate that we use the same word to refer to a serious mental illness.

Depression affects about one in five men at some time in our lives. But what does being 'depressed' really mean? When is being down is a sign of illness, rather than normal sadness, frustration, anger or stress?

There are a few pointers. Firstly, doctors don't consider depression to be 'clinical' - meaning an illness in need of treatment - unless symptoms have lasted for more than two weeks.

This kind of depression is caused by changed brain and body chemistry, which doesn't happen overnight. In fact, depression often comes on so slowly that many men have difficulty noticing the change.

Although depression is technically a mental illness, many of the symptoms of depression are actually physical. Depressed people find it difficult to get to sleep, or to feel rested when they wake.Waking through the night, or waking far too early in the morning is quite a strong sign of depression.

Depressed people have little energy, and they often find it difficult to motivate themselves to do anything. Appetite and interest in food diminishes, and many people lose weight. Aches and pains and a general physical malaise are also very common.

Thinking is affected too. For example, it can be difficult to concentrate, so that it's impossible to follow a TV programme all the way through. Everything seems an effort.

Even short newspaper articles are a struggle, and few depressed people can manage to read novels, even if this is something they used to enjoy. Memory also suffers, so that it's hard (for example) to remember phone numbers, and easy to forget important things at work.

Other, subtler, changes in thinking can be harder to spot. For example, people find that minor setbacks - things they would usually shrug off - seem devastating. Problems in one area generalise, so that they seem to affect every aspect of life.

In severe depression, people can feel that they are useless; that their lives are a waste of time; or that they must have been guilty of something terrible. A few people hear voices that aren't really there (hallucinations).

It's very common to feel you can't face the day, to wish you didn't have to wake up in the morning, or even to feel like ending it all. Suicidal thoughts like this are a frightening symptom - fortunately, most people don't act on them.