Because happiness is a subjective state, the only way to directly measure happiness is to ask people how they feel. There may be physiological indications of positive mood, such as low levels of stress hormones, but there are no objective measures of happiness per se. Self-report measures pose a number of problems, however. People may not always know exactly how they feel. Alternatively, they may bias their reports according to how they want to see themselves or what they feel is socially desirable. Nonetheless, self-reports of happiness have yielded meaningful data in a large body of research. Researchers also distinguish between rating overall happiness (How happy are you in general?) and tracking moment-by-moment emotional states. The second type of rating allows researchers to connect emotional reactions to the specific activities people are engaging in at the time.