Being born in the summer could give you a sunnydisposition for life. And a winter birthday might cast a permanent shadow over your happiness, scientists believe.
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 intriguing 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 mimic the short days of winter.
After they were weaned, they were kept in the same light cycle for several weeks or switched to the opposite one.
Finally, they were plunged 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 seasonal affective disorder.'
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 disorders such as seasonal depression, bipolar depression and schizophrenia - all of which are associated with disruption of normalcircadian rhythms.'
But being born in the summer can have drawbacks - 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 pesticides during the first few months of pregnancy - a critical time for brain development.
(Read by Renee Haines. Renee Haines is a journalist at the China Daily Web site.)
(Agencies)
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。 实验表明，出生的季节会极大地影响人体生物钟运转的速度。 冬天出生的人体内的生物钟运转得太慢，从而可能对健康和性格产生影响。 这一离奇有趣的理论来自一项对老鼠的研究，该研究把刚出生的老鼠暴露在不等量的光线下长达数月。 有些老鼠被放置在夏天的光照条件下，每天有16小时的"白天"和8个小时的"黑夜"。其他老鼠则只有8个小时的"白天"，和冬天昼短夜长的光照条件类似。 在这些老鼠断奶后，他们被放置在相同的光照周期中长达数周，或转换到相反的光照环境。 最后，科学家把它们都放置到黑暗中，观察它们的反应。 根据《自然神经科学》杂志的报道，那些在夏天的光照条件下成长起来的老鼠像往常一样作息，而那些在缺少光线的环境中成长起来的老鼠在应对这一变化时却显得吃力。 研究者道格拉斯•麦克马洪教授说："在冬天的光照周期中成长起来的老鼠对于季节的变化表现出夸大的反应，这和得了季节性情感抑郁症的人类病患的反应极其相似。" 此外，这一研究发现还提出了一个非常有趣的可能性，就是人的大脑在生命的最初几周或几个月所感知到的光照量可能会对心情造成影响。 教授说："我们体内的生物钟对白日的长短进行衡量，并根据季节变化而改变我们的行为。 "几项研究都表明，出生在冬天的人发生情绪异常的风险更大，他们更容易患上诸如季节性抑郁症、两极型忧郁症和精神分裂症这种病，而这些病症都和打乱正常的昼夜节律有关。" 但是，出生在夏天也有不利之处，比如近视的风险会增大。 另外一项面向美国150多万儿童的研究显示，夏天受孕后生出的小孩不如其他小孩聪明。 据认为，这是因为这些小孩在孕期的头几个月接触更多的杀虫剂所致，而这几个月正是大脑发育的关键时期。
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