Energy

Many distinct meanings of the word "Energy" have come into popular use, either before the modern scientific concept of energy was fully developed, or later by analogy to it.

Psychological Energy
Many humans actions can be scientifically explained in terms of energy. Thus, humans maintain their metabolism when they exercise or do mechanical work. But the word "energy" is also used in a casual sense as a synonym for psychological motivation, creativity, agitation, excitement, or responsiveness. "Fatigue" or a "lack of energy" can be either a result of expending physical energy, or a psychological condition brought on by an excess of intellectual activity, intense emotional experiences, inadequate sleep, or an imbalance or natural fluctuation in hormones and neurotransmitters. Emotional "exhaustion" is thought to be brought on not by a lack of physical energy (in the scientific sense described above), but by a particular chemical state in the brain. Low motivation is associated with both malnutrition (including insufficient calories - a unit of physical energy - in the diet) and elevated blood glucose (sugar) levels (which provides lots of physical energy, but which has other chemical effects that influence mood). Drugs such as depressants, stimulants (present in some energy drinks and dietary supplements), and sedatives directly affect these types of emotional states. Some mental disorders such as mania, bipolar disorder, and depression involve improper regulation of brain chemicals which determine mood.

Sigmund Freud, among others, developed a notion of psychic energy which is similar to, but distinct from, the general scientific notion of energy. This psychological model, while popular, is no longer considered to be a scientifically accurate of how the mind works.