The Holiday Happiness Index study, which was conducted in collaboration with Natalie Dattilo-Ryan, a Harvard-affiliated clinical psychologist and holistic happiness expert, looked at 50 destinations throughout the world and ranked them across five "feel good" factors, including sunlight, quality sleep, a healthy diet, time in nature, and physical exercise. To determine which are the sunniest, the study also took into account each place's total hours of sunlight per year.