They can be startlingly vivid and real, or fragmented and bizarre.
Whatever form they take, dreams can reveal a lot about what a person really thinks.
"Dreams give us an X-ray of our unconscious. They show us, particularly when we're at life's passages or turning points, what's going on inside, what we're having trouble coping with," said Alan Siegel, a professor at the University of California at Berkeley who has studied dreams for nearly 30 years.
Lisa Oppenheim has had the same dream all her life. She recently embarked on a quest to find out what it means.
"I am on these jugs, these clay pots that are suspended in space and darkness," she said. "Beneath me there's a sea … there are killer whales in the sea. And I need to jump from jug to jug [to] avoid falling in the water."
Oppenheim has had the dream whenever she was about to move to a new home or a new city. While she doesn't know exactly what causes the dream, she has a theory about why it occurs when she's going to relocate.
"It's my subconscious preparing itself for transition," she said. "And to me, at this point in my life, it's more comforting than scary."
Experts say Oppenheim's situation is common. Disturbing dreams often occur around major life events: moves, marriage, divorce, the death of a loved one or pressure at work.
Siegel believes stressful or upsetting dreams can be a sign of what's buried deep inside the mind.
"Dreams are important to survival. They are crucial to the balance of our psyche. … They help us work out conflicts that we're having every single day," he said. "They go to the cutting edge of what's stressful, what's bothering us and what we're working out in our lives."
Siegel said that dreams can provide guidance if people understand them. Psychologists recommend recording dreams in a journal so that they can be examined for clues later.
"The two best techniques are to focus on the emotion in the dream, and that's often the most important part, and just ask yourself what in my waking life feels like this," said Deirdre Barrett, an assistant professor of psychology at Harvard Medical School.
Barrett also recommends trying to connect details in the dreams to one another.
"You can just go through and associate detail to detail," she said. "This particular dark figure in the dream, did they look like anyone I've seen recently?"
Siegel and Barrett explained the meaning behind common dreams.
Being Stuck or Paralyzed
"This dream usually has to do with some form of emotional paralysis," Siegel said. "You're stuck in some way in your life and the dream is reflecting that in a metaphor."
Being Naked or Dressed Inappropriately
"These dreams seem to crop up when people are feeling some kind of social inadequacy," Barrett said.
"[This could be] a feeling of rejection, being fired, things not working out with a friend or colleague," Siegel said. "When you experience emotional or physical loss, that's when you tend to have the teeth falling out dream."
Nightmares often reflect an especially intense emotion or unresolved conflict. "The stronger the emotion in the dream, whether it's pleasure or fear, the more likely the dream is important," Barrett said.
For more information on dreams, visit the International Association of Studies of Dreams.