World happiness levels are at their lowest level in over a decade, with the number of people who say they feel stressed and worried rising, according to a survey published on Wednesday.

Conflict-hit Central African Republic (CAR) was the world's unhappiest place last year, with Iraq in second place, according to the ranking by pollsters Gallup.

"Collectively, the world is more stressed, worried, sad and in pain today than we've ever seen it," the group's managing editor, Mohamed Younis, wrote in a foreword to the study.

Gallup surveyed more than 154,000 people in 146 countries on whether they had felt pain, worry, stress, anger or sadness the previous day. It said the global mood was at its gloomiest since the first such survey in 2006.

Image: 2018 Global Emotions Report

Sub-Saharan Africa led the way, with 24 of 35 countries surveyed reaching a 10-year happiness lows in 2017, often due to civic unrest crippling healthcare systems and causing people to go hungry.

"In CAR and some of these other places, high percentages of the population are just struggling to afford the basics," the study's lead author, Julie Ray, told the Thomson Reuters Foundation by phone.

CAR has been ravaged by violence, with most of the country now beyond the control of the government, and about three in four residents said they experienced pain and worry.

Wealthier countries were not immune to the dip in mood. About half the Americans interviewed said they were stressed – roughly the same proportion of respondents as in the CAR.

Economist Jan-Emmanuel De Neve said it was "disturbing" to see the global mood souring against a backdrop of rising wealth and material progress.

"There is probably a more structural indicator around the increasing wealth not being inclusive enough," said De Neve, an associate professor at the University of Oxford who has written about the link between income and happiness.