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A mood is an emotional state. Moods differ from emotions in that they are less specific, less intense, and less likely to be triggered by a particular stimulus or event. Moods generally have either a positive or negative valence. In other words, people typically speak of being in a good mood or a bad mood.
Mood also differs from temperament or personality traits which are even longer lasting. Nevertheless, personality traits such as optimism and neuroticism predispose certain types of moods. Long term disturbances of mood such as clinical depression and bipolar disorder are considered mood disorders. Mood is an internal, subjective state but it often can be inferred from posture and other behaviors. "We can be sent into a mood by an unexpected event, from the happiness of seeing an old friend to the anger of discovering betrayal by a partner. We may also just fall into a mood." 
Etymologically, mood derives from the Old English mōd which denoted military courage, but could also refer to a person's humour, temper, or disposition at a particular time. The cognate Gothic mōds translates both θυμός "mood, spiritedness" and ὀργή "anger".
Sleep is a major factor in one's mood. If a person is sleep deprived they could become more irritable, angry, more prone to stress, and less energized throughout the day. "Studies have shown that even partial sleep deprivation has a significant effect on mood. University of Pennsylvania researchers found that subjects who were limited to only 4.5 hours of sleep a night for one week reported feeling more stressed, angry, sad, and mentally exhausted. When the subjects resumed normal sleep, they reported a dramatic improvement in mood." Generally, evening oriented people, as compared to morning ones, show decreased energy and pleasantness and heightened tension 
Depression, chronic stress, bipolar disorder, etc. are considered mood disorders. It has been suggested that such disorders result from chemical imbalances in the brain's neurotransmitters, however some research challenges this hypothesis.
Like positive moods, negative moods have important implications for human mental and physical wellbeing. Moods are basic psychological states that can occur as a reaction to an event or can surface for no apparent external cause. Since there is no intentional object that causes the negative mood, it has no specific start and stop date. It can last for hours, days, weeks, or longer. Negative moods can manipulate how individuals interpret and translate the world around them, and can also direct their behavior.
Negative moods can affect an individual’s judgment and perception of objects and events. In a study done by Niedenthal and Setterlund (1994), research showed that individuals are tuned to perceive things that are congruent with their current mood. Negative moods, mostly low-intense, can control how humans perceive emotion-congruent objects and events. For example, Niedenthal and Setterland used music to induce positive and negative moods. Sad music was used as a stimulus to induce negative moods, and participants labeled other things as negative. This proves that people's current moods tend to affect their judgments and perceptions. These negative moods may lead to problems in social relationships. For example, one maladaptive negative mood regulation is an overactive strategy in which individuals over dramatize their negative feelings in order to provoke support and feedback from others and to guarantee their availability. A second type of maladaptive negative mood regulation is a disabling strategy in which individuals suppress their negative feelings and distance themselves from others in order to avoid frustrations and anxiety caused by others' unavailability.
Negative moods have been connected with depression, anxiety, aggression, poor self-esteem, physiological stress and decrease in sexual arousal. In some individuals, there is evidence that depressed or anxious mood may increase sexual interest or arousal. In general, men were more likely than women to report increased sexual drive during negative mood states. Negative moods are labeled as nonconstructive because it can affect a person’s ability to process information; making them focus solely on the sender of a message, while people in positive moods will pay more attention to both the sender and the context of a message. This can lead to problems in social relationships with others.
Negative moods, such as anxiety, often lead individuals to misinterpret physical symptoms. According to Jerry Suls, a professor at the University of Iowa, people who are depressed and anxious tend to be in rumination. However, although an individual's affective states can influence the somatic changes, these individuals are not hypochondriacs.
Although negative moods are generally characterized as bad, not all negative moods are necessarily damaging. The Negative State Relief Model states that human beings have an innate drive to reduce negative moods. People can reduce their negative moods by engaging in any mood-elevating behavior (called Mood repair strategies), such as helping behavior, as it is paired with positive value such as smiles and thank you. Thus negative mood increases helpfulness because helping others can reduce one's own bad feelings.
Positive mood can be caused by many different aspects of life as well as have certain effects on people as a whole. Good mood is usually considered a state without an identified cause; people cannot pinpoint exactly why they are in a good mood. People seem to experience a positive mood when they have a clean slate, have had a good night sleep, and feel no sense of stress in their life.
There have been many studies done on the effect of positive emotion on the cognitive mind and there is speculation that positive mood can affect our minds in good or bad ways. Generally, positive mood has been found to enhance creative problem solving and flexible yet careful thinking. Some studies have stated that positive moods let people think creatively, freely, and be more imaginative. Positive mood can also help individuals in situations in which heavy thinking and brainstorming is involved. In one experiment, individuals who were induced with a positive mood enhanced performance on the Remote Associates Task (RAT), a cognitive task that requires creative problem solving. Moreover, the study also suggests that being in a positive mood broadens or expands the breadth of attentional selection such that information that may be useful to the task at hand becomes more accessible for use. Consequently, greater accessibility of relevant information facilitates successful problem solving.
Positive mood has also been proven to show negative effects on cognition as well. According to the article "Positive mood is associated with implicit use of distraction", "There is also evidence that individuals in positive moods show disrupted performance, at least when distracting information is present". The article states that other things in their peripheral views can easily distract people who are in good moods; an example of this would be if you were trying to study in the library (considering you are in a positive mood) you see people constantly walking around or making small noises. The study is basically stating that it would be harder for positive moods to focus on the task at hand. In particular, happy people may be more sensitive to the hedonic consequences of message processing than sad people. Thus, positive moods are predicted to lead to decreased processing only when thinking about the message is mood threatening. In comparison, if message processing allows a person to maintain or enhance a pleasant state then positive moods need not lead to lower levels of message scrutiny than negative moods. It is assumed that initial information regarding the source either confirms or disconfirms mood-congruent expectations. Specifically, a positive mood may lead to more positive expectations concerning source trustworthiness or likability than a negative mood. As a consequence, people in a positive mood should be more surprised when they encounter an untrustworthy or dislikable source rather than a trustworthy or likable source.
"Psychologists have found that even if you’re in a bad mood, you can instantly lift your spirits by forcing yourself to smile."  Numerous amounts of research studies have shown that making a facial expression, such as smiling, can produce effects on the body that are similar to those that result from the actual emotion, such as happiness. Paul Ekman and his colleagues have studied facial expressions of emotions and have linked specific emotions to the movement of specific facial muscles. Each basic emotion is associated with a distinctive facial expression. Sensory feedback from the expression contributes to the emotional feeling. Example: Smiling if you want to feel happy. Facial expressions have a large effect on self-reported anger and happiness which then affects your mood. Ekman has found that these expressions of emotion are universal and recognizable across widely divergent cultures.
The idea of social mood as a "collectively shared state of mind" (Nofsinger 2005; Olson 2006) is attributed to Robert Prechter and his socionomics. The notion is used primarily in the field of economics (investments).