Not only can eating an apple a day keep the doctor away, but new studies show it can also improve your mood as well.
A study published in the British Journal of Health Psychology found a strong day-to-day relationship between more positive mood and higher fruit and vegetable consumption, but not other foods.

“On days when people ate more fruits and vegetables, they reported feeling calmer, happier and more energetic than they normally did,” says Dr Tamlin Conner, a Department of Psychology researcher from Otago’s Department of Human Nutrition.
Along with colleagues Bonnie White, and Dr Caroline Horwath, Dr Conner studies 281 young adults who were asked to keep a daily food diary for21 consecutive days. Each day participants logged how they felt using nine positive and nine negative adjectives rated numerically. They were also asked five questions about what they had eaten that day. Specifically, participants were asked to report the number of servings eaten of fruit (excluding fruit juice and dried fruit), vegetables (excluding juices), and several categories of unhealthy foods like biscuits/cookies, potato crisps, and cakes/muffins.
“After further analysis we demonstrated that young people would need to consume approximately seven to eight total servings of fruits and vegetables per day to notice a meaningful positive change. One serving of fruit or vegetables is approximately the size that could fit in your palm, or half a cup. My co-author Bonnie White suggests that this can be done by making half your plate at each meal vegetables and snacking on whole fruit like apples,” says Dr Conner.