On average, 20% to 25% of your sleep is a dream state. For most people, that’s more than two hours of dreaming every night.

Researchers have found that Americans have more physically aggressive dreams than people from most other industrialized nations.

While dreams of falling, being chased or being in school are common, dreams about having sex are not as frequent.

Sex dreams make up about the same amount of both men's and women's dreams, according to a University of Montreal study. The study's author noted that "sexual intercourse was the most common type of sexual content, followed by sexual propositions, kissing, and fantasies."

When researchers presented sleeping volunteers with odors of rotten eggs or roses, the subjects who smelled rotten eggs reported more bad dreams while the subjects who smelled roses reported more good dreams. Researchers speculate that odors have a stronger effect on dream emotions than other external stimuli because the sense of smell is linked directly to parts of the brain associated with dreaming.

While some researchers believe dreams are merely a byproduct of sleep, others think dreams are important for memory consolidation, conflict resolution, and regulation of mood.

Some medications can affect the central nervous system and cause nightmares. These include antidepressants, narcotics, and barbiturates.

In sleep studies, women tend to report having more nightmares than men. It's unclear, however, if women are just better at recalling their dreams than men or if they, in fact, have more nightmares.

You're no more likely to die during a dream about dying than at any other time. Dreams of dying are fairly common.

You may not be able to control your dreams, but there are things you can do to increase your dream retention, including getting enough sleep, limiting drug and alcohol intake, keeping a "dream journal,” and training yourself to write down your dreams immediately after you wake up.

For the most part, dreams are similar for people from different cultures around the world, but there are variations.

Pregnant women dream more often than average and tend to have greater recall of their dreams. Experts say this is caused by changes in hormone levels during pregnancy and because pregnant women sleep more, and thus dream more, than usual.

Everyone dreams. According to the National Sleep Foundation, humans spend more than two hours dreaming each night.

Studies have shown that people who were born blind do not have visual dreams. People who go blind in early childhood are also unlikely to have much visual imagery in their dreams. But people who go blind in later childhood, or as adults, do dream visually.

Frederik Van Eeden, a Dutch physician, coined the term “lucid dream” in 1913 to identify dreams in which someone is aware that he or she is dreaming.

Dreams occur during both REM (rapid eye movement) and non-REM sleep, but sleep studies show that brain activity is heightened during REM periods.

Most people have recurring dreams at some point in their lives. Experts say these dreams may be more significant than ordinary dreams and could point toward a significant issue in your life. But recurring dreams may be something to be concerned about if the content is disturbing and leads to sleep deprivation.