Low Mood / Sadness / Depression
We all feel sad at times, sometimes more than others. Sadness can be a useful emotion in signalling to us that we are in a state of perceived lack or loss of something important to us, and in motivating us to obtain it if possible, or to identify alternative sources of reinforcement.
However, when we feel down or sad very often, including periods of feeling very sad that last for prolonged amounts of time, this can be the psychophysiological syndrome known as depression, with hypothesized causes including brain chemistry (e..g, low serotonin), brain electrical activity (e.g., higher power of slow brainwaves than of fast brainwaves in the left hemisphere of the brain), as well as causes of these chemical and electrical processes (e.g., negative thoughts, lack of reinforcement from the environment).
Some people experience depression not as feeling down, but as just not feeling "up," not enjoying things they used to enjoy.
Depression can involve cognitive, emotional, behavioral and physiological aspects, including:
Cognitive: Cognitive content can include negative thoughts about the self, world and future; helplessness; and hopelessness. Cognitive process can include difficulty concentrating, as well as "depressive rumination" in which we obsess about something negative.
Emotional: Sadness or anger. The psychodynamic school says that sadness is anger turned inward. Another way to look at this is that anger and sadness can be sequential responses to frustration at the blocked attainment of a goal.
The behavioral school has a model called 'learned helplessness," by which when a goal is frustrated and we believe we cannot influence it anymore, we shut down rather than continue what we think would be wasting our time and energy. Here, frustration of an important goal first brings about anger or anxiety which motivates us to try harder to achieve the goal (fight or flee). However, once we realize, or think, that we won't succeed, we then feel helpless, and can become depressed.
As sadness can feel worse than anger (as sadness, in a sense, results from admitting defeat, as opposed to continuing to fight or flee) we sometimes "flip" the sadness into anger. Children, for example, often manifest sadness as anger, which may indicate that adults who do so are falling back into earlier overlearned modes of thinking, feeling and behaving.
Behavioral: When we are depressed, we don't feel like doing anything. We may isolate ourselves from others at home, at work and socially; we may stop doing things that we usually enjoy or find useful, such as reading or going to the gym. We lack motivation. We don't think anything will make us feel happy, so we do the bare minimum, more out of fear of losing everthing than out of expecting whatever is gained to bring contentment. We become behaviorally de-activated.
Physiological: When depressed we may sleep much more than usual with difficulty getting out of bed in the morning, or we may have insomnia, with difficulty falling asleep or staying asleep. Sleeping too much puts the brain in a slow-wave mode similar to depression. Sleep deprivation leads to fatigue and more depression. We may have little appetite and lose weight, or we may have an urge to binge on comfort foods and gain weight. We may feel energetically sluggish and even walk and move more slowly than usual.