I don't mean to overstate the reliability of suicide rates for happiness. A variety of factors may influence them. However, there are reasons to believe they correlate with happiness.

What is a good measure of happiness? Virtually all measures are deeply flawed, but the most reliable is probably: Intra-observer self-report in situations when signalling is unlikely. People are probably decent at knowing when they were happier or less happy within their own lives. Inter-observer reports are far harder to justify.

People report being happier when they live in places with adequate sunshine; inadequate sunshine correlates with increased suicide rates. People report being happier when they are not facing loss of job, public humiliation, divorce, and a number of other events; these events correlate with increase suicide rates. People report being happier when mental illness symptoms are reduced (particularly depression); people with mental illness (particularly depression) have a higher suicide rate.

Obviously, suicide rates are not a perfect proxy for happiness... but I cannot find a more reliable easily-measured statistic.