Being born in the summer could give you a sunny disposition¹ for life. And a winter birthday might cast a permanent shadow over your happiness, scientists believe.

科学家认为，出生在夏天将赋予你开朗的性情，而出生在冬天则可能给你的快乐蒙上永久的阴影。

Experiments suggest that season of birth dramatically affects the way the speed at which the body clock ticks.

A winter birth could leave it moving too slowly – affecting health and personality.

The intriguing²（有趣的） theory comes from a study of mice exposed to varying amounts of light in the first months of life.

Some were given summer conditions of 16 hours of light and eight hours of darkness per day. Others had only eight hours of daily light, to mimic³ the short days of winter.

After they were weaned（断奶） , they were kept in the same light cycle for several weeks or switched to the opposite one.

Finally, they were plunged⁴ into darkness and watched to see how they would react.

Those reared in summer conditions kept to a daily routine, but those brought up in little light struggled to cope with the change, the journal Nature Neuroscience reports.

Researcher Professor Douglas McMahon said: ‘The mice raised in the winter cycle show an exaggerated response to a change in season that is strikingly similar to that of human patients suffering from seasonal⁵ affective disorder⁶.’

Despite this, the finding raises the intriguing possibility that the amount of light to which the human brain is exposed in the first weeks or months of life affects mood.

The professor said: ‘Our biological clocks measure the day length and change our behaviour according to the seasons.

‘Several studies show that people born in the winter months have an elevated risk of mood disorders⁷ such as seasonal depression, bipolar（两极的） depression and schizophrenia（精神分裂症） – all of which are associated with disruption of normalcircadian rhythms.’

But being born in the summer can have drawbacks（缺点） – such as an increased risk of short-sightedness.

Another study of more than 1.5 million children in the US clearly showed that those conceived in the summer are less clever than other youngsters.

It is thought that they receive more exposure to pesticides⁸ during the first few months of pregnancy⁹ – a critical time for brain development.