The most in-depth study on human life has found that relationships play a big part in human well-being. The Harvard Study on Adult Development has been interviewing and collecting data from many generations of participants since 1938, checking in on the participants from time to time, asking them to fill out questionnaires, and watching them fall in and out of their relationships. Relationships are usually not the priorities of most people, but it can impact our productivity, sensitivity, immune system, sleep, and mood. When asked about their relationships, some participants would try to avoid the questions or not turn in the questionnaires, because they did not want to think about loneliness and other difficult aspects in life. Loneliness is difficult to measure and is subjective. Research has found that as people get older, they start cherishing their relationships most, and that for older adults, loneliness is more dangerous than obesity. Although reflecting may be hard, it can help us plan out the direction we wish our relationships to go, make a plan, and meet up with people we hold dear. Spending time with them, maintaining the relationships, yields great benefits for our happiness and health. "Even small investments today in our relationships with others can create long-term ripples of well-being."