Flow experiences, also known as optimal experiences, occur when a person's body or mind is fully engaged in a challenging activity that they find meaningful. A landmark study called the Experience Sampling Study found that people reported being happiest when absorbed in work or goals. Flow is described as a state of total focus and enjoyment in an activity where nothing else seems to matter. Key elements of flow include clear goals, immediate feedback, a balance of challenge and skills, merging of action and awareness, focus without distractions, lack of worry, absence of self-consciousness, and distorted sense of time where the activity is intrinsically rewarding.