January is the peak time every year for depression. One cause is 'seasonal affective disorder' or SAD, which is put down to lack of sunlight. Light keeps us happier than darkness, so SAD is often treated with artificial sunlight.

But there is much more to depression in wintertime, or at any other time, than just SAD. At a party, there's always the odd person in a corner, looking miserable. The party atmosphere removes our inhibitions about showing our feelings in public. It makes many people happy, but others slip into very different moods. Some get maudlin, others angry and aggressive, yet others anxious, and a few become morose and dejected.

These differences are not just due to 'character' or 'circumstances'. Depression, anxiety, aggression, agitation, calmness and happiness are not just our reactions to what is happening around us, or to our personal problems. Life isn't as simple as that. There are many people whose moods are not appropriate to what is going on around them, and who can't change their mood, even though things are changing for the better - or worse.

Our mood is not under our control. A sizeable minority of us tend to be depressed. Some people are almost emotionless, while others are easier to rouse into anger than the rest. Some are permanently anxious. There are even people who are unrealistically happy and optimistic when they are faced with disaster.

New ways of examining the brain are showing why we are all so different. They highlight the parts of the brain that are active in different emotional states. They can identify which chemical substances - called neurotransmitters - pass between brain cells during different emotions. They show striking differences between the brains of people with and without mood disturbances like severe depression or anxiety.

An extreme example is from a young woman with schizophrenia, who was having terrifying visions. They were connected with unusual activity in a part of the brain not normally linked to the eyes, proving that they were the result of abnormal chemical and electrical activity in that area. Given the appropriate drugs to damp down this activity, her visions ceased, and she returned to normal.

One person in six becomes depressed at some time, and one in 20 has severe long-term depression.

Depression, anxiety and other mood changes often exist together, along with bodily symptoms like headaches, indigestion and bowel problems: they are all the result of neurotransmitter or other chemical imbalances in the brain. If we correct the chemistry the mood problems should be solved.

This is already happening. New anti-depressant prescription drugs have been designed to correct specific neurotransmitter imbalances. They have complex names, such as selective serotonin reuptake inhibitors (SSRIs) or selective noradrenaline reuptake inhibitors (SNRIs). Others are on the way.

These drugs do work. But they take a few weeks to show much effect, so that people with severe depression should be well looked after and encouraged to keep on taking the pills in the first few weeks while they are still feeling low. They also have side effects that should be explained before they are taken.

This applies to any age group - depression strikes children as well as adults, and can start for the first time after middle age. It is also occurs alongside other illnesses such as diabetes, heart disease, cancer, infections and arthritis. Such depression is not just an emotional response to the illness, but a biochemical change that should respond well to treatment. People with chronic physical illnesses who are also depressed do less well than people with no mood problems. Lifting their depression does not cure their other illness, but they feel far better and can fight it more effectively.

The message from all this is not the old clich 'it's all in the mind'. But it surely is 'all in the brain'. I'm not sure that we know yet what the mind is - but brain research is getting us closer to it.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.