It is 7:45 p.m. in Beijing, and the sun is setting on a long summer’s day. Two thousand miles west in Ürümqi, the official time on the clock is identical, but there will be another two hours of daylight. Work tomorrow morning starts at the same time in both cities.

In China, just one time zone spans the entirety of the landmass. Working hours, TV schedules and markets stick resolutely to the time in Beijing, but daylight does not.

It has been widely noted that people living on the eastern frontier of a time zone tend to sleep more than those living on the west. Falling asleep tends to be a function of both the time on the clock and the amount of daylight, while waking up is more likely driven by the obligations of work or school. This means people tend to stay awake later in the west of a time zone (where sunset is later) but get up at the same time as everyone else (when the alarm clock demands). Since there is a widely observed impact of sleep on health, you might, by extension, expect to see a relationship between longitude and life expectancy.

Indeed, on the face of it, the picture is clear-cut. The chart below shows China’s 30 largest cities, split into groups according to longitude. The bar on the left shows the average life expectancy of the most westerly cities across to the most easterly cities on the right (raw source data can be found here).

There is a fairly clear trend toward longer lives in the east, implying a correlation between longitude and life expectancy. Using just a map and a calculator, you can estimate life expectancy to within couple of years for most Chinese cities. Put another way, you would gain one month of additional life expectancy for every 40 miles east you live from the western border.

The full picture is, inevitably, a little more complex, due to lurking variables. A lurking variable is a factor in a correlation that, if left unchecked, can lead us to draw the wrong conclusion. A classic example of a lurking variable is the apparent relationship between ice-cream sales and murder rates. The correlation is real, but the causation is not. Because in practice, it is high temperatures that drives both.

In our example, wealth is the lurking variable. Economic development is not uniform across China and, on average, eastern regions have higher incomes. There is evidence that sleep drives both health and income. So it could be argued that time zone and its effect on sleep is ultimately impacting income. But realistically, that is unlikely to be the case here. The economic development of China’s eastern coastal regions predates the influence of the single time zone (implemented in 1949) by a thousand or so years.

There are certain things that we know are true: Longitude impacts sleep, and sleep impacts health. But a single national time zone also drives economic development and, given the evident impact of income on life expectancy, the net effect is likely to be positive. And the good news is that people living in China today can expect to live 30 years longer than in than those living in 1949.

While sleep, and by extension, health can be a casualty of living in the far west area of a time zone, the other good news is that this ultimately comes down to personal choice. There are two apt maxims on the subject: “日出而作, 日落而息” (get up at sunrise and go to bed at sunset), but perhaps for our friends in the west this would better be replaced with “早睡早起身体好” (early to bed, early to rise).