Psychologists have long emphasized the importance of child’s play in brain development. But now, research suggests that grown-ups need their playtime, too.

What does that mean, exactly? Scholars don’t agree on a definition of play. But in general, it involves any activity freely chosen for the joy of it, rather than for an extrinsic reward like money. When we think of play, social activities like basketball or Scrabble might first to come to mind, but play can be solitary, too. Even gardening and knitting can count. Some consider daydreaming play, too, a sort of “default state for the idling mind,” says Scott Eberle, editor of The American Journal of Play.