Stress is an unavoidable part of life. In some circumstances it can be useful; stress can be motivating, and it can prepare us mentally and physically for difficult situations. But it can also be crippling if the source of the stress is not addressed. It can also result in long-term mental health problems: anxiety, low self-esteem and depression, amongst others.

Stress serves an important purpose in that it prepares the body and mind to deal with an immediate challenge - in evolutionary terms, typically a physical threat such as escaping from danger or fighting for our safety. It was not intended to last for long periods of time. In fact, some of the ways that stress prepares the body for short-term survival can actually become harmful if prolonged. Stress compromises the immune system and redirects the body's resources away from other vital systems in order to prioritise others (like the muscles) in the immediate future. Physically, prolonged stress can have serious health consequences: anything from aches and pains, migraines, digestion problems (including irritable bowel syndrome) to worse conditions such as circulatory problems and even some types of cancer.

Mentally, too, ongoing stress has serious effects. Stress changes the way we think in order to help us make immediate decisions for our safety. In the long term, the mentality that this encourages (simple "all-or-nothing" thinking) can emphasise repeated failures, leading to low self-esteem and depression.

More than this, stress changes the brain's chemistry and activity. Stress activates an area of the brain known as the Hypothalamic-Pituitary-Adrenal (HPA) axis, which co-ordinates the response to the stressor. Interestingly, however, neurological studies have shown that depression also involves heightened activity in the HPA axis. In other words, in terms of brain activity, stress and depression don't look so very different.

This is hardly surprising in some ways. After all, the pass/fail mentality that stress pushes us into explains the link to depression when the problem continues unaddressed. But also, depression does serve an evolutionary purpose. When an animal loses status - or fails to cope adequately with a source of stress - it actually makes sense that it should not act the way it did before. Subdued behaviour, retreating, hiding away, sleeping more and conserving energy all make sense in such circumstances. They are ways of surviving in a world that has, for one reason or another, recently become more hostile. There's no reason to think that human brain chemistry should be any different. Neurologically, depression looks a lot like a habitual, ingrained stress response.

What this means in practice is that stress should be treated as a warning signal. Some stress can be productive and useful, but if it is extreme or prolonged, it can lead to serious and long-term depression. It's far better to address the stressor when you have the chance than to wait until depression strikes and you don't have the choice.

Guy Brandon is a counsellor, author and founder of www.StressingOut.org, a website dedicated to resources for stress, depression, anxiety and other related problems.

For more information, see http://www.StressingOut.org/depression/.