Based on a global ranking of happiness levels across 156 countries, Finland has claimed the No. 1 spot in this year's World Happiness Report.

Now in its sixth year, the World Happiness Report is produced by the United Nations Sustainable Development Solutions Network.

The organization, along with three economists from Columbia University, the University of British Columbia and the London School of Economics' Center for Economic Performance, created the report using data from the Gallup World Poll to reveal which countries are happy and why.

The report was released on March 14, less than a week before the United Nations celebrates World Happiness Day on March 20.

This year, the United States ranked No. 18 — falling four spots from last year and five from two years ago — "in part because of the ongoing epidemics of obesity, substance abuse and untreated depression," according to World Happiness Report co-editor and Columbia University professor Jeffrey Sachs.

Over the past two years, the world's top 10 happiest countries have remained the same, but have slightly shuffled positions. Through a measurement of happiness and well-being called the "Cantril ladder," Gallup asked nationally representative populations to value their lives on a scale from 0 to 10, with the worst possible life valued at 0 and the best valued at 10.

The top countries frequently have high values for all six of the key variables that contribute to overall well-being: income (GDP per capita), healthy life expectancy, social support, freedom, trust (absence of corruption) and generosity.

Here are the top 10 happiest countries and how they measure on the Cantril ladder: