Foods greatly influence all aspects of overall health.  A poor diet especially one with a lot of processed foods can lead to depression.   The brain has chemicals called neurotransmitters which are the regulators of mood.  The neurotransmitters associated with mood are dopamine, serotonin, and norepinephrine.  When the brain produces serotonin, tension is eased.  When it produces dopamine or norepinephrine, we tend to think and act more quickly are are generally more alert. At the neurochemical and physiological level, neurotransmitters are extremely important.  These substances carry impulses between nerve cells.  Serotonin, for example, plays a role in mood, seep, and appetite.  Low levels of serotonin can lead to depression, anxiety, and sleep disorders.  The substance that processes serotonin is the amino acid tryptophan.  The consumption of tryptophan increases the amount of serotonin mad by the brain.  Thus, eating complex carbohydrates(not simple carbohydrates such as fructose, sucrose, and lactose), which raise the level of tryptophan in the brain(thereby increasing serotonin production) has a calming affect.  High-protein foods on the other hand, promote the production of dopamine and norepinephrine, which promote alertness.