A new research study of people in the U.S. and UK shows that once we reach middle age, we start growing happier.

The study, led by medical researchers at the University of Warwick, analyzed the lifestyle and health of more than 10,000 people and found respondents reported better mental quality of life as they aged.

Despite decline in their physical bodies, people from cross-cultural backgrounds living under differing health care systems similarly  reported becoming happier once they moved beyond their 40's.

This supports previous research, for example by University of Warwick Professor Andrew Oswald, which suggests that happiness levels follow a U-shape curve with their lowest point in the mid-40s.

"We suggest that this could be due to better coping abilities, an interpretation supported by previous research showing older people tend to have internal mechanisms to deal better with hardship or negative circumstances than those who are younger," said Dr Saverio Stranges, who led the study.

"It could also be due to a lowering of expectations from life, with older people less likely to put pressure on themselves in the personal and professional spheres."

The study also looked at the effect of sleep on quality of life, and found there was an optimum window of sleep duration.

Those who sleep between six and eight hours per day tended to have both better physical and mental health scores than those who slept on average less than six hours or more than eight hours.

Owing to its cross-cultural nature, the study was also able to identify differences between the quality of life of US and UK respondents.

In the US, respondents' social background was more likely to affect their quality of life, with those in higher socio-economic groups reporting better mental and physical quality of life. The researchers suggested this could be due to the presence of universal healthcare in the UK, which has a levelling effect on well-being.

Cross-cultural Comparison of Correlates of Quality of Life and Health Status: The Whitehall II Study (UK) and the Western New York Health Study (US), was published in the European Journal of Epidemiology.

The study, led by medical researchers at the University of Warwick, analyzed the lifestyle and health of more than 10,000 people and found respondents reported better mental quality of life as they aged.

Despite decline in their physical bodies, people from cross-cultural backgrounds living under differing health care systems similarly  reported becoming happier once they moved beyond their 40's.

This supports previous research, for example by University of Warwick Professor Andrew Oswald, which suggests that happiness levels follow a U-shape curve with their lowest point in the mid-40s.

"We suggest that this could be due to better coping abilities, an interpretation supported by previous research showing older people tend to have internal mechanisms to deal better with hardship or negative circumstances than those who are younger," said Dr Saverio Stranges, who led the study.

"It could also be due to a lowering of expectations from life, with older people less likely to put pressure on themselves in the personal and professional spheres."

The study also looked at the effect of sleep on quality of life, and found there was an optimum window of sleep duration.

Those who sleep between six and eight hours per day tended to have both better physical and mental health scores than those who slept on average less than six hours or more than eight hours.

Owing to its cross-cultural nature, the study was also able to identify differences between the quality of life of US and UK respondents.

In the US, respondents' social background was more likely to affect their quality of life, with those in higher socio-economic groups reporting better mental and physical quality of life. The researchers suggested this could be due to the presence of universal healthcare in the UK, which has a levelling effect on well-being.

Cross-cultural Comparison of Correlates of Quality of Life and Health Status: The Whitehall II Study (UK) and the Western New York Health Study (US), was published in the European Journal of Epidemiology.