In the fairy tale "Goldilocks and the Three Bears," the heroine declares one bowl of porridge tastes too hot, another too cold, and the third — well, that's just right.

Ever since, the idea that avoiding extremes provides the best results has been called the Goldilocks Principle. Today, researchers from New Zealand's Otago University are advocating it for flourishing emotionally.

Looking at the habits and mental health of young adults, they discovered that too much or too little of a good thing damages well-being. The study, published in the journal Frontiers in Psychology, found that sleeping eight hours nightly produced the highest level of well-being. Sleeping less than eight or more than 12 hours a night was indicative of depression.

Diet and physical activity also follow the Goldilocks Principle. Study participants who ate 4.8 servings a day of raw fruit and veggies had the best quality of life. Those who ate fewer than two servings or more than eight servings had the lowest measure of well-being and mental health.

And physical activity — at least 30 minutes daily, but not more than two continuous hours some days — defeats inflammation caused by being sedentary and doesn't cause chronic inflammation from overdoing it.

Getting the right serving size of any one of these lifestyle factors helps offset deficiency or excess in the others.

But the study found that integrating all three into daily life can increase your well-being by more than 35% and decrease your risk for symptoms of depression by 38%.

And that, as Goldilocks said, is just right.