A data story

The chapter ends with a data story. Some animals sleep as little as 2 hours a day; others as much as 20. Humans usually sleep 7 to 9 hours, but this varies quite a lot from person to person.

How much sleep a person needs depends on their age, living conditions and lifestyle, including the food they eat and the things they do. You will have noticed that babies sleep much longer than adults.

The book plots the usual sleep time of Indians from age 6 to age 75. Read it as before: first what is given, then what it means.

At age 6, the average is about 9.5 hours a day. Through the teenage years and into adulthood the line falls steadily, reaching about 8 hours somewhere between ages 30 and 50.

Then it turns. After 50, daily sleep time rises again, to about 8.5 hours. So sleep does not simply go down with age: it dips in middle age and then comes back up.

Notice that this graph looks different from the earlier ones. The temperature graph was a few straight segments joining twelve monthly points. This one looks like a smooth curve.

That is because it has about 80 data points, packed so closely that the segments between them are too short to see separately. Nothing is different in principle; there are just many more points.

A line graph is the right choice here. A column graph of the same data would need about 70 columns: crowded and heavy, and the shape of the trend would be lost. A line is light and shows the pattern at once. Then let the graph raise questions. Why do newborns sleep so long? Do people in other countries sleep differently? Do animals show the same pattern through their lives?