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Consider how these factors from modern life commonly influence us:

Being indoors (lack of sunlight): Reduced melatonin
Stress: Reduced serotonin
Modern food processing: Fatty acid imbalances and chemical sensitivities
High sugar/carbohydrate and low-fat diet: More insulin; prostaglandin imbalance
Lack of exercise: Decreased serotonin and dopamine
Lack of sleep: Reduced serotonin
Poor nutrition: Reduced serotonin
Boring classes/job, lack of activity: Reduced dopamine & norepinephrine.
Deionized air: Reduced serotonin

Reduced serotonin levels from stress, lack of sleep & exercise, poor nutrition, and lack of sunlight are connected with ADD, irritability, depression, aggression, anxiety, lack of concentration, chronic pain, restlessness or fatigue, nausea, obsessive-compulsive disorder, weight gain or loss, fibromyalgia, arthritis, chronic fatigue syndrome, heat intolerance and other syndromes. Fluctuating serotonin levels are connected with bipolar disorder (manic depression) and hypomania. You don't have to feel depressed or anxious; symptoms may be purely physical.

Reduced dopamine/norepinephrine caused by boring surroundings and lack of exercise may manifest as ADD, impulsivity, lack of concentration, restlessness, and depression or loss of pleasure. Dopamine is the "feel good" chemical which illegal drugs mimic (such as cocaine, heroin, marijuana) as well as cigarettes, coffee and alcohol.