"Groggy," in its strictest dictionary definition, means "dazed," although the term is most often used to refer to a strong state of mental and physical fatigue. The word is also used interchangeably with "drowsy," which pertains to mental lethargy and sleepiness. An individual usually feels groggy either when sleep-deprived or when just waking. Grogginess can be heightened after consumption of alcohol or certain drugs. Certain psychological and medical disorders can also result in chronic grogginess.
Someone who feels groggy usually has trouble thinking clearly, as the brain is in a state of fatigue. Common behavior in groggy individuals includes mumbling, sluggish movement, and lapsing into mental blanks. In some cases, people can fall into microsleeps, uncontrollable brief periods of sleep lasting a few seconds, as a result of grogginess. Another key characteristic of being groggy is a dazed look in the person's eyes, as though half-awake.
The word "groggy" finds its origins in grog, a alcoholic drink enjoyed by seamen in the 1700s. Sailors who had too much grog were often dazed, lethargic, and sluggishly drunk. In addition, they would be hungover in the morning, often slow to react to any sort of stimuli. As a result, sailors caught in this state were playfully called "groggy."
Although alcohol is believed to be the first identified cause of grogginess, the term has come to be known more as a state associated with sleep deprivation. Individuals with extremely active lifestyles often end up overexerting themselves and get insufficient sleep. When the lack of sleep catches up with their systems, their brains tend to function at slower paces in an effort to conserve energy. This explains why individuals are often groggy after days of little to no sleep or after waking from a very short sleep.
Certain medications have been known to induce grogginess. These include "drowsy" formulations of cold medicines and sleeping aids. Commercially-available sedatives like diazepam depress the central nervous system, with strong doses causing grogginess in patients. Illegal drugs such as opium, though mainly used to induce states of euphoria, can also result in grogginess, especially after the user's "high" wears off.
Grogginess can come about as a symptom of depression, as chemical imbalances in the brain tend to slow down its function. Patients suffering from insomnia also report feeling groggy, as they cannot fall asleep despite being tired. Grogginess can be due to vitamin deficiencies as well. An inadequate supply of vitamin B12, for example, can cause individuals to wake up feeling lethargic.