We spend about a third of our lives and during that time we dream. Many theories exist to explain why we dream, but researchers are still uncertain. Some people believe that dreams have symbolic meanings, while others believe that they are related to waking life.

What scientists do know is that most people dream while they sleep, and those dreams can be fascinating, exciting, scary, or simply outlandish. Here are 10 things to know about dreams. 9 common dreams and their meanings
We dream every night
The brain is active throughout the night. Brain activity in the forebrain and midbrain is especially intense during rapid eye movement (REM) sleep, which is when we dream.

Adults and babies dream about two hours a night, even if they don't remember it. In fact, researchers have found that people often have multiple dreams each night, each lasting between 5 and 20 minutes.2

In a typical life, people spend an average of six years dreaming.

5 common characteristics of all dreams

We forget most of our dreams
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7 simple theories about why we dream
We forget up to 95% of all our dreams as soon as we wake up.

One theory as to why dreams are so difficult to remember is that changes in the brain during sleep do not support the processing and storage of information needed to form memories. Brain scans of sleeping people have shown that the frontal lobe - an area that plays an important role in memory formation - is not active during REM sleep, the stage in which dreams occur.4

We can also forget our dreams due to altered levels of certain neurotransmitters, especially acetylcholine and norepinephrine, during REM sleep.5

Another study found a link between dream recall and medial prefrontal brain (MPFC) material density. Participants with higher white matter density reported higher dream recall.

Not all dreams have color
Most people reported dreaming in color, but about 12% of people reported only dreaming in black and white.

In studies in which dreamers were awakened and asked to choose colors that matched the people in their dreams, people often chose softer pastels.

People under the age of 25 rarely report black and white dreams. However, people over 55 years old report black and white dreams about 25% of the time.7 The researchers believe this difference is a result of childhood exposure to black and white television. This idea is supported by an older study, which found that people in the 1940s rarely reported dreaming in their dreams.