Some dreams feel so real and vivid that it stays in the mind long after one wakes up. Most dreams, however, are immediately forgotten.

Scientists have carried out investigations to figure out why some dreams are memorable while others disappear.

What Happens When People Dream

Dreaming usually takes place during the state of sleep known as the Rapid Eye Movement or REM. This part of the sleep cycle is also sometimes referred to as desynchronized sleep because it mimics signs of being awake.

"Everybody dreams every night-but people who tend to remember their dreams more often may be waking up during the REM phase of sleep, which is where dreams with narrative content occur," said Susana Martinez-Conde, a professor at the State University of New York, in a conversation with Gizmodo last year. "If you have dreams in the middle of the night but then go on to have other phases of sleep without waking up, then those dreams are mostly going to be inaccessible to you."

Another factor is the content of the dream. Deirdre Barrett, an assistant professor at Harvard Medical School, mentioned in the same Gizmodo story a previous study that found nonsensical narrative and jumbled imagery are a lot harder to grasp and, therefore, more difficult to remember. Dreams that have a clear structure are more likely to be retained in the mind.

Barrett also noted that when people wake up, the brain will only begin to turn on some functions. A study in 2011 found that the hippocampus, which is critical for moving information from short-term to long-term memory, is one of the last regions of the brain to go to sleep. It is also one of the last to wake up.

"So, you could have this window where you wake up with a dream in your short-term memory, but since the hippocampus is not fully awake yet, your brain is not able to keep that memory," explained Thomas Andrillon, a neuroscientist at Monash University, in a separate story published by Live Science.

How To Remember Dreams

Robert Stickgold, a sleep researcher from Harvard Medical School, explained to the BBC that morning routines cause dreams to fade away. He shared that after waking up, people who want to retain their dreams should lie still, not open their eyes for a while, and "float" while trying to remember.

Another simple but effective way for people to remember their dreams is to tell themselves that they want to remember as they slowly drift to dreamland.