Sleep is foundational to our health. A large body of data links poor sleep to heart disease, stroke, diabetes, and depression. There’s also evidence that unhealthy sleeping habits may lead to sexual dysfunction and that sleep quality is associated with mood.
The National Institutes of Health recommends between seven and nine hours of sleep each night for healthy adults. But new research suggests that this standard may not hold true across the globe; in fact, this research suggests that cultural norms and expectations play a role in how much sleep people need for optimal health.
First, it’s important to understand the wide variety of sleep practices around the world. People living in Japan have some of the shortest sleep durations, averaging only seven hours per night; they also have a cultural practice of inemuri, short naps in public or while at work. People in Spain and Italy also average only seven hours of sleep per night, but they have a cultural tradition of a longer mid-afternoon nap (known as a siesta in Spain or a Riposo in Italy). Meanwhile, the Dutch and French average more than eight hours of sleep per night while people in Bulgaria and South Africa average more than nine.
Which of these is best for optional health? A new systematic review published in the Proceedings of the National Academies of Sciences seeks to answer that question. Researchers combined sleep-tracking data from studies conducted in 70 countries and compared them with national health statistics such as rates of heart disease, obesity, diabetes, and overall life expectancy. When calculating sleep duration, they combined the duration of naps and nighttime sleep.
Surprisingly, they found that average sleep duration for each country was not a reliable predictor of health. Countries with shorter average sleep durations, such as Japan, did not have higher rates of illness or lower life expectancies compared to countries with higher average sleep durations. In fact, longer average sleep durations were associated with higher rates of obesity at a national level.
To further understand this phenomenon, the researchers conducted a second study, surveying nearly 5,000 participants from 20 countries about their sleep patterns, health, and cultural sleep norms.
At the individual level, the study confirmed what researchers expected: People who reported that they slept an optimal number of hours also reported better health. Individuals who slept at levels they personally perceived to be too little or too much reported worse health, which is consistent with earlier sleep studies.
But the data highlighted an interesting twist: The amount of sleep associated with the best health varied in each country. In countries with shorter national averages of sleep duration, the amount of sleep required for optimal health was lower; in countries with longer average sleep duration, the amount of sleep required for optimal health was higher. In each country, the optimal amount of sleep for health was always higher than the national average of sleep duration. On the whole, people who had sleep patterns closer to their national average were more likely to report better health. For example, people in Japan who averaged about seven hours of sleep per night reported optimal levels of health, while those who reported significantly less or more sleep per night reported lower-than-optimal health.
The finding highlights a phenomenon sociologists call “cultural fit”: Essentially, when an individual’s beliefs and behaviors line up with societal norms, they may receive psychological or even physiological benefits.
The study did have some limitations. Participants self-reported sleep and health data, which is not always accurate. While the study controlled for variables such as socioeconomic status, nutrition, and geography, other variables could have skewed the results. But even with these limitations, the study provides new insights into how culture may influence sleep and health.
The take-home message: New research suggests that cultural expectations play a role in the connections between sleep and health.
