Adrenal gland, or the suprarenal gland -- Either of two small endocrine glands, one located above each kidney, consisting of the cortex, which secretes several steroid hormones, and the medulla, which secretes epinephrine.

Analgesic -- A medication that reduces or eliminates pain.

Circadian rhythms -- The natural rhythms regulating the body's internal mechanisms for rest and sleep. These rhythms continue unabated regardless of whether or not the body is exposed to morning daylight or plunged into nighttime darkness.

Conjugation -- A chemical bonding in which the product is easily broken down into the original compounds. It is one of the primary processes by which the body metabolizes narcotics and other substances.

Deamination -- The removal of the amino group, NH2, from an organic compound. It is one of the primary processes by which the body metabolizes narcotics and other substances.

Demethylation -- The removal of the methyl group, CH3, from a compound. It is one of the primary processes by which the body metabolizes narcotics and other substances.

Epinephrine or adrenalin -- A hormone secreted by the adrenal medulla and released into the bloodstream in response to physical or mental stress. It initiates many bodily responses, including the stimulation of heart action and an increase in blood pressure, metabolic rate, and blood glucose concentration.

Hydroxylation -- The introduction of hydroxyl (hydroxide) into an ion or radical, usually by the replacement of hydrogen. It is one of the primary processes by which the body metabolizes narcotics and other substances.

Melatonin -- A hormone derived from serotonin and secreted by the pineal gland. It stimulates color change in the epidermis of amphibians and reptiles and plays a role in sleep, aging, and reproduction in mammals.

Monoamine oxidase (MAO) -- An enzyme that deaminates monoamines, amine compounds that function as neurotransmitters which contain one amino group, through oxidation. MAO functions in the nervous system by breaking down monoamine neurotransmitters.

Narcotic -- A frequently addictive depressant drug (e.g., opium and derivatives such as morphine) that in moderate doses dulls the senses, relieves pain, and induces profound sleep, but in excessive doses causes stupor, convulsions, or coma. Also used in reference to other drugs, such as LSD or marijuana, subject to similar legal restrictions whether physiological addictive or not.

Neurotransmitters -- Chemical substances, such as acetylcholine or dopamine, which transmit nerve impulses across a synapse.

Norepinephrine or noradrenaline -- A substance that is both a hormone and neurotransmitter. It is secreted by the adrenal medulla and the nerve endings of the sympathetic nervous system to cause constriction of blood vessels and increases in heart rate, blood pressure, and the sugar level of the blood.

Pineal Gland, pineal, pineal body, or pineal organ -- A small body that ascends from the top of the third ventricle of the brain and is enclosed by the pia mater, a fine vascular membrane that envelops the brain and spinal cord. It primarily functions as an endocrine gland that produces melatonin.

Reuptake -- The reabsorption of a neurotransmitter, such as serotonin or norepinephrine, by a neuron following impulse transmission across a synapse.

Serotonin -- An organic compound formed from the amino acid tryptophan and found in human tissue, especially the brain, blood serum, and gastric mucous membranes. It is active as a neurotransmitter, in the constriction of blood vessels, stimulation of the smooth muscles, and regulation of cyclic body processes.

Sympathetic amines -- A group of drugs, including caffeine, benzedrine, amphetamine, mescaline, and ephedrine, which mimic the chemical behavior of hormones in the nervous system.

Synapse -- The junction across which a nerve impulse passes from an axon terminal to a neuron, muscle cell, or gland cell.