Sleep apnea occurs in all age groups and both sexes but is more common in men (it may be underdiagnosed in women) and possibly young African Americans. It has been estimated that as many as 18 million Americans have sleep apnea. Four percent of middle-aged men and 2 percent of middle-aged women have sleep apnea along with excessive daytime sleepiness. People most likely to have or develop sleep apnea include those who snore loudly and also are overweight, or have high blood pressure, or have some physical abnormality in the nose, throat, or other parts of the upper airway. Sleep apnea seems to run in some families, suggesting a possible genetic basis.