The idea that something out there in space can affect our health down here on Earth is one of the oldest in medicine. Influenza, for instance, literally means the 'influence' of certain baleful planetary forces; the word lunatic comes from the idea that the mad were particularly susceptible to the moon. However, for modern medicine, it is an approach that smacks too much of pseudo-science so is now largely dismissed.

But an intriguing paper in the British Journal of Psychiatry last month claims to have found evidence linking geomagnetic storms in the earth's upper atmosphere with admissions to hospital for severe depression.

Geomagnetism is the science concerned with the earth's magnetic field and the effect on the field of cosmic rays, the penetrative rays from outer space.

Dr Ronald Kay, a consultant psychiatrist at Westbank Day Hospital in Falkirk, compared the records of these storms, which are strongly associated with solar flares, and the admission rates to Lothian psychiatric hospitals over a 10-year period. He found that two weeks after a storm there was a significant rise in male, but not female, admissions for psychotic depression.

Although these storms occur all the year round - the most dramatic sign of them is the Aurora Borealis in far northern latitudes - they reach a peak around the time of the spring and autumn equinoxes. 'Ever since Hippocrates, doctors have known that depression gets worse around April and September,' says Dr Kay. Many in the field now agree that lack of light contributes to the onset of winter depressions, known as seasonal affective disorder, or SAD.

But that does not explain the rise in depression in April. Poets talk in terms of love and rebirth at the beginning of the year: 'Spring can really hang you up the most,' observes the songwriter Fran Landesman, and T S Eliot declared: 'April is the cruellest month.' For many, the onset of spring signals change and it is the time when people tend to do stressful things, such as get married or move house.

Dr Kay believes that both peaks in depressive illness are more fundamentally influenced by the seasonal disturbances in the earth's geomagnetic field.

How might such a connection work? 'We already have a model from the work done on SAD,' he says. 'The key here is the pineal gland in the brain, which produces the chemical melatonin in response to light. Melatonin is involved in regulating our activity cycle. The highest levels of melatonin in humans are found in sleep and in some animals during hibernation.'

It is not clear whether SAD may, in the end, be found to relate to too much or too little melatonin. At this stage Dr Kay believes it may relate to changes in levels of the chemical, as affected by the seasons.

'People with SAD are particularly sensitive to the reduction of the amount of light in winter,' he says. 'They respond to this reduction by producing more melatonin and, more important, by their melatonin-producing cycle becoming out of sync with the rest of the body. Non-depressives can experience a sense of this when they get jet-lag.'

So why the difference between men and women? 'Melatonin seems to be linked with regulating sexual functions. Its concentration varies during puberty and also during the menstrual cycle. It could be that female hormones provide some defence against its effects or it could simply be that males' response to severe depression is more violent and that they are therefore admitted to hospital more quickly.'

Dr Kay points out that there is considerable evidence from work on animals to show that the pineal gland responds to geomagnetic fields in a similar way as it does to light. In the laboratory, scientists can test for the effects of geomagnetism by eliminating all other visual and sensual influences.

'It is possible to affect animals' daily rhythms by exposing them to geomagnetic fields (GMF). For example, there is evidence that homing pigeons seem to use GMF to navigate. GMF can also affect body functions such as cell activity.

'We don't as yet know how geomagnetic storms affect the general population. It might be that they just trigger off people who are susceptible or it could be that their effect ranges from making people feel mildly irritated to full-blown depression.'

Dr Kay admits that he is working at the edge of legitimate science. 'My colleagues did think it was a bit odd when I first got involved in this area. There have been lots of connections made between sun-spot cycles and everything from the rise and fall of the stock markets to the quality of Burgundy vintages, and it's all pretty speculative. But as the research started coming in, they became much more interested.'

Should Dr Kay's findings be confirmed, they may provide another reason for the famous Scandinavian angst - geomagnetic activity is at its most intense at the poles. It might also mean that before long we will receive geomagnetic storm warnings along with the pollen count and sunburn advice on the national weather bulletins.