Dreams are stories and images that our minds create while we sleep. They can be entertaining, fun, romantic, disturbing, frightening, and sometimes bizarre.

This article looks at how we dream, what nightmares are, lucid dreams, and why some dreams are difficult to remember where others are more memorable.

How do we dream?

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Why and how we dream remains something of a mystery.

Sleep happens in cycles. Each complete sleep cycle takes about 90 to 110 minutes.

Most dreams happen during a phase known as rapid eye movement (REM) sleep. The first REM sleep period usually occurs around 70 to 90 minutes after we fall asleep.

During this phase, an amino acid known as glycine is released from the brain stem onto the motor neurons. These motor neurons conduct impulses outward from the brain or spinal cord.

This release of glycine effectively causes the body to become paralyzed.

This paralysis is believed to be nature's way of making sure we do not act out our dreams and thus prevents injury.

The first sleep cycles each night contain relatively short periods of REM and long periods of deep sleep. As the night progresses, REM sleep periods increase in length, while deep sleep decreases.

Researchers have different theories about the relationship between dreaming and REM sleep. Does REM-sleep physiology explain the dream experience? Or is it not necessary to be in REM sleep for dreaming to occur?

One study has suggested that dreaming can happen during both REM and non-REM (NREM) sleep, but that different physiological processes underlie the dreaming at each stage.

The dreams that occur during these periods may differ significantly in both quality and quantity and probably result from different processes.

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Paralysis during the REM stage of sleep could ensure we do not act out our dreams.

Visual imagery appears to be more common after waking from REM sleep, compared with NREM sleep. People reported visual images after 83 percent of REM awakenings, compared with only 34 percent after stage 2 sleep.

One study has proposed that the hormone cortisol plays an important role in controlling memory systems during sleep. High cortisol levels have been observed late at night and during REM sleep.

Cortisol affects the interaction between the hippocampus and the neocortex. This interaction appears to have an impact on a specific type of memory consolidation. These could affect the content of dreams.

In NREM sleep, the interaction between the neocortex and the hippocampus is not disrupted, and typical episodic memories occur.

However, in REM sleep, dream content reflects only neocortical activation. Dreams are more likely to be fragmented and bizarre.

What are bad dreams and nightmares?

Both adults and children can experience bad dreams and nightmares.

During a nightmare, the dreamer may experience a range of disturbing emotions, such as anger, guilt, sadness or depression. However, the most common feelings are fear and anxiety. The person usually wakes up at least once during the dream.

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Nightmares can cause distressing emotions and can be especially disturbing for children.

Causes of bad dreams include:

  • stress
  • fear
  • trauma
  • emotional issues
  • medication or drug use
  • illness

A study that looked at 253 episodes described as "nightmares" found they frequently contained:

  • physical aggression
  • bizarre and emotionally intense situations
  • failures and unfortunate endings

One in three of these nightmares contained primary emotions other than fear.

In a further 431 bad dreams, as opposed to nightmares, interpersonal conflicts were common. Just over half contained primary emotions other than fear.

In another study, 840 German athletes discussed distressing dreams that occurred on the nights before an important competition or game.

About 15 percent of the athletes reported having had at least one distressing dream before an important competition in the last 12 months. Most of these related to athletic failure.

Elsewhere, a survey in which 30 women who were dealing with relationship violence described their dream experiences, half reported having weekly nightmares, and just over half had recurring dreams.

Dream events included:

  • drowning
  • being chased
  • bring killed
  • killing someone else

One theory about recurrent dreams is the threat simulation theory. According to this theory, dreams are an ancient biological defense mechanism that aims to repeatedly simulate threatening events, presumably to prepare people for threats they might face in their waking life.

Researchers have proposed that children who live in a threatening environment would dream more actively than those who do not, and at least one study has confirmed this.

In one investigation, children who had undergone severe trauma experienced a significantly greater number of dreams and a higher number of threatening dream events, featuring more severe threats, compared with children who had not experienced trauma.

However, in a study that looked at the dreams of 190 school children aged 4 to 12 years who had not undergone any trauma, the following was noted:

  • Fear featured in 75.8 percent of dreams.
  • Worries featured in 67.4 percent.
  • Scary dreams represented 80.5 percent.

Fears relating to scary dreams were common among children aged 4 to 6 years old, and more so among those aged 7 to 9 years. These fears became less frequent between the ages of 10 and 12 years.

Types of fears, worries, and dreams changed across age groups. Fears and scary dreams relating to imaginary creatures reduced with age, while worries about test performance increased with age.

In one study, dream reports from 610 teenagers showed that disturbing and normal dreams occur at both 13 and 16 years of age. However, disturbing dreams are especially common among adolescent girls.

Girls who often had disturbing dreams were also more likely to show signs of trait anxiety, even at 13 years of age.

Nightmare triggers

Certain conditions appear to increase the frequency of nightmares in some people.

These include:

Migraine: Recurrent dreams featuring complex visual imagery, often terrifying nightmares, can occur as migraine aura symptoms. These dreams often involve the emotions of fear and anguish.

Sleep apnea: People with sleep apnea have more emotionally negative dreams than those who simply snore while asleep.

Depression: Frequent nightmares are associated with suicidal tendency in individuals with major depression.

Night or sleep terrors

Night terrors are different from nightmares.

A child who is experiencing night terrors may:

  • scream
  • shout
  • thrash around
  • panic
  • jump out of bed
  • fail to recognize parents trying to comfort them

Night terrors occur on waking abruptly from deep NREM sleep, while nightmares are thought to happen during REM sleep.

Around 1 to 6 percent of children are thought to experience sleep terrors at some point in their childhood. It is common in children aged between 3 to 12 years. Children are not fully awake in these episodes, even if their eyes are open, and they usually have no memory of the event the next day.

The episodes usually occur in the early part of the night and can continue for up to 15 minutes.

Night terrors are more common in children with a family history of night terrors or sleepwalking behavior.

A night terror attack may be triggered by anything that:

  • increases the amount of deep sleep the child has, such as tiredness, fever or certain types of medication
  • makes the child more likely to wake from deep sleep, such as excitement, anxiety, or sudden noise

Most children will eventually grow out of night terrors.

Research has suggested that parasomnias and other sleep conditions — such as restless leg syndrome (RLS) and sleep-disordered breathing — may run in families. There could be a genetic link.

Night terrors have also been linked to enlarged tonsils and adenoids.

What are recurring dreams?

A recurring dream is a type of dream that occurs on a regular basis when we sleep.

A study of 212 reports of recurring dreams found that:

  • Two in three dreams contained one or more threats, which tended to be dangerous and aimed at the dreamer. When facing a threat, the dreamer tended to take defensive or evasive actions that were possible and reasonable.
  • Fewer than 15 percent of the recurrent dreams depicted realistic and probable situations. In these, the dreamer rarely succeeded in fleeing the threat, despite efforts.

What are lucid dreams?

Lucid dreaming is a rare state of sleep in which the dreamer knows they are dreaming and they gain insight into their state of mind during the dream.

Research has shown that during lucid dreaming, parts of the brain are active that are normally suppressed during sleep. Findings have suggested that that lucid dreaming is a unique state of consciousness separate from any other mental state.

Scientists have found that certain cortical areas that are activated during lucid dreaming.

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Lucid dreams usually occur while a person is in the middle of a regular dream and suddenly realizes that they are dreaming.

A study of lucid dreams in school children and young adults revealed that:

  • lucid dreaming is "quite pronounced" in young children
  • the incidence drops at around16 years of age

The study authors proposed a link between the natural occurrence of lucid dreaming and brain maturation.

What are wet dreams?

A wet dream is when an ejaculation occurs during sleep, usually during a sexual dream. The person may not remember the dream, and it can happen without touching the penis. They may or may not wake up.

They usually affect boys during puberty, when the body starts to produce the male hormone testosterone. Once the body can produce testosterone, it can release sperm.

Wet dreams are a normal part of growing up and cannot be prevented. Some boys may have several dreams a week, while other never experience one. This, too, is normal.

Impact of drugs and health conditions

The use of some medications can affect dreaming.

Antidepressants and SSRIs

A review of small studies has reported that selective serotonin reuptake inhibitors (SSRIs) may intensify dreaming.

Results showed that:

  • People both with and without depression experienced a decrease in dream recall frequency when using antidepressants.
  • More positive dream emotions were linked to tricyclic antidepressant use.
  • Nightmares occurred after discontinuing tricyclic antidepressants and the monoamine oxidase inhibitors (MAOIs) phenelzine and tranylcypromine.
  • Both starting and discontinuing the use of SSRIs or SNRIs seem to intensify dreaming.

Anesthetic use

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Since the introduction of anesthesia, hallucinations and dreams that are blurred with reality have often been reported.

Dreams and hallucinations have long been linked to sedation under anesthesia.

Sexual hallucinations have, in the past, led to allegations of sexual molestation or assault by medical doctors or professional nursing staff.

The following drugs have been linked to dream experiences:

Propofol: People who have been given this anesthetic drug have reported hallucinations and dreams that are "pleasant" and may have sexual connotations. The dreams might also involve uninhibited behavior or verbal expression of intimate thoughts.

Ketamine: Volunteers who took a sub-anesthetic dose of ketamine experienced more dream unpleasantness over 3 nights than those who took a placebo.

Alcohol: People who were undergoing detoxification following alcohol dependence experienced poorer quality sleep and more negatively toned dreams compared with healthy controls. After 4 weeks of abstinence, both sleep quality and dream experience improved slightly. During this time, the participants with alcohol dependency dreamed significantly more often about alcohol than the group that had not experienced dependency.

Studies have found that subjective sleep and dream quality is strongly impaired in patients with alcohol dependency.

Marijuana and cocaine

Sleep disturbances and unpleasant dreams have been linked to cocaine withdrawal, and difficulty sleeping and strange dreams have been reported after discontinuing tetrahydrocannabinol (THC), or marijuana, use.

Health conditions that affect dreams

Some health conditions can change a person's sleep and dream quality.

Psychotic major depression

People with affective and non-affective psychoses have been found to have higher levels of unusual thinking, or cognitive bizarreness, both when dreaming and awake.

Narcolepsy

Narcolepsy with cataplexy (NC) is a neurological disorder that features excessive daytime sleepiness and changes in sleep patterns.

Studies have found that most people have about 85-percent dream recall, whether or not they have NC. However, people with NC have reported longer and more complex first-REM dreams.

These findings suggest that for people with NC, the cognitive processes underlying dream generation operate more effectively earlier in the night, compared with other people.

Parkinson's

Sleep disturbances and bad dreams have been linked to Parkinson's disease.

One study looked at the relationship between testosterone levels, violent dreams, and REM sleep behavior disorder (RBD) in 31 men with Parkinson's disease (PD).

Results suggested that those with RBD were more likely to experience violent dreams, but that neither RBD nor violent dreams was linked to testosterone levels in men with PD.

Another study involved both men and women with PD. It linked RBD with violent dreams in both sexes. The dream content was similar for male and female participants, but men tended to experience more violent dreams.

Post-traumatic stress disorder

Disturbed sleep patterns, nightmares, and anxiety-filled dreams are symptoms of post-traumatic stress disorder (PTSD).

Remembering dreams

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There is something about the phenomenon of sleep that makes it difficult to remember what has occurred. Most dreams are forgotten unless they are written down.

It is often said that 5 minutes after the end of a dream, we have forgotten 50 percent of its content, and 10 minutes later, we have forgotten 90 percent. Dream researchers estimate that around 95 percent of all dreams are forgotten entirely upon awakening.

Some people have no difficulty remembering several dreams nightly, while others rarely or never recall dreams. Some aspect of sleep appears to make it difficult for dreamers to remember what happened.

Most dreams are forgotten, but sometimes a dream is suddenly remembered later in the day or on another day. Writing down or recording dreams may help you remember them. This suggests that the memory is not totally lost, but for some reason it is hard to retrieve.

How does the brain affect dream memories?

Brain lesion and neuroimaging studies have indicated that the temporo-parieto-occipital junction and ventromesial prefrontal cortex play crucial roles in dream recall.

Surface EEG studies showed that sleep cortical oscillations associated with successful dream recall are the same as those involved in forming and recalling episodic memories while awake.

Cortical brain oscillations of human sleep appear to predict successful dream recall.

Specific cortical activity has been linked with successful dream recall after waking up from REM sleep, a finding which strengthens the theory that dream recall and episodic memory during wakefulness are linked.

A different area of the brain has been linked with successful dream recall after awakening from stage 2 NREM sleep.

Overall, these findings suggest that mechanisms underlying the encoding and recall of episodic memories may remain the same across different states of consciousness, in other words, whether awake or asleep.

Another study using MRI techniques found that vivid, bizarre, and emotionally intense dreams — the dreams people usually remember — are linked to parts of brain areas known as the amygdala and hippocampus.

The amygdala plays a primary role in the processing and memory of emotional reactions. The hippocampus has been implicated in important memory functions, such as consolidating information from short-term to long-term memory.

Scientists have also identified where dreaming is likely to occur in the brain.

People who have a clinical condition known as Charcot-Wilbrand syndrome lose the ability to dream.

A loss of the ability to dream was also noted in one person who experienced a lesion in a part of the brain known as the right inferior lingual gyrus. This is located in the visual cortex. It may be that this area of the brain, which is associated with visual processing, emotion and visual memories, plays a role in either generating or transmitting dreams.

People have speculated about dreams for thousands of years, but only recently have advances in technology make it possible to study brain activity in ways that may help us understand what really happens when we dream. However, much about the life of dreams remains a mystery.