Research has shown that unhappiness is associated with poor health, and poor health is associated with mortality. But does unhappiness itself directly influence mortality?

The answer is no, according to a study published in The Lancet in December 2015.

“Illness makes you unhappy, but unhappiness itself doesn’t make you ill. We found no direct effect of unhappiness or stress on mortality, even in a ten-year study of a million women,” said Bette Liu, principal investigator and corresponding author of the study.

Scientists compiled pre-existing data from the Million Women Study, which recruited women from 1996 to 2001 and follows them electronically to track health data and causes of death.

In the current study, which contained data from 719,671 women, the research team sorted mortality into three categories: death from all causes, death from heart disease, and death from cancer.

“Our…study shows no robust evidence that happiness itself reduces cardiac, cancer or overall mortality,” said Liu.

Because so many factors have been proven to affect—or at least occur alongside—happiness, scientists adjusted for each variable. Some of these factors included physical activity, having a partner, being a nonsmoker, involvement with religious or other group activities, and adequate sleep (around 8 hours; much more or less sleep produced the opposite effect).

Conversely, data was also adjusted for factors that contribute to unhappiness, such as clinical depression, anxiety and poor physical health.

After taking these variables into account, scientists found that happiness alone does not seem to affect mortality.

“[The data] showed some excess mortality to be associated with unhappiness, but this was completely eliminated after additional adjustment for personal characteristics and for poor health,” reported Liu.

“By far the most important adjustment factor was self-rated health. A…review of previous studies has confirmed that self-rated health predicts an increased risk of death,” she added.

A major limitation of the study is the way it measured happiness: a single question asking participants to rate their happiness on a four-point scale.

“There is no perfect or generally agreed way to measure happiness,” said Liu.

Future research may focus on ways to accurately measure happiness that can be used across multiple studies.