Introduction

Definition of sleep talking (somniloquy) 

Sleep talking, known by some as somniloquy, is a very common occurrence even in healthy individuals, with some individuals having larger genetic predispositions. It can happen in all stages of sleep, however, the person who is talking during their sleep is rarely aware of it.¹

Stages of sleep

To better understand the topic of sleep talking, it is important to know the differences between the stages of sleep.

During our sleep, and even when we are awake, the brain produces electromagnetic waves of different frequencies, which can be measured by machines such as EEGs (electroencephalograms). By changes in wave frequencies, it is possible to determine in which stage of the sleep cycle someone is (See Figure 1). We differentiate between the following stages of sleep:

Figure 1: The sleep cycle over the period of one night ⁴

Stage 1 – light sleep (Non-REM)

• In this stage, theta waves (4-7 Hz) start dominating over the alpha waves that are present when we are awake (8-13 Hz)
• This makes up 5% of the sleep cycle
• Physical effects: light muscle tension, regular breathing^3,4

Stage 2 – deep sleep (Non-REM)

• In this stage, theta waves are dominant (4-7 Hz)
• Sleep spindles can be seen, which are brief, powerful outbursts of brain activity, believed to be related to memory consolidation (See Figure 2)
• K complexes are also recorded, which arelong delta waves, lasting approximately 1 second, and believed to be important for staying asleep, as well as memory consolidation (See Figure 2)
• This makes up 45% of the sleep cycle
• Physical effects: Lower heart rate, drop in body temperature, teeth grinding^3,4

Figure 2: K-complex and sleep spindle recorded by EEG²

Stage 3 – deepest sleep (Non-REM)

• Delta waves dominate (low frequency, high amplitude); therefore, this stage of sleep is also known as slow wave sleep (SWS)
• It is hard to wake up people who are in this stage of sleep
• In this stage the body repairs, grows, and strengthens the immune system
• Sleepwalking, night terrors, and bedwetting occur in this stage
• 25% of sleep cycle; however, the time spend in this stage decreases with age
• This stage is, in some texts, divided into stage 3 and 4 of SWS^3,4

Stage 4 – REM sleep

• REM means rapid eye movement, which is a characteristic sign of REM sleep
• This means that people in this stage of sleep rapidly move their eyes under their eyelids, almost as if they were awake
• Even the brain activity is almost as if a person were awake, as beta waves are typical for this stage of sleep
• In actuality, the skeletal muscles tend to be paralysed during this stage
• This is believed to be because the brain practices memories of physical activities, so the muscles are paralysed to prevent the person dangerously moving while asleep
• People tend to experience vivid dreams and nightmares in this stage
• This makes up 25% of sleep cycle^3,4

When does sleep talking occur, and how does it differ across different stages of sleep? 

Now that you understand how the different stages of sleep work, and what are good for, we can dive into the topic of sleep talking, which can be present in all stages of sleep.⁵ However, there are still differences in how people sleep talk in REM compared to non-REM sleep. In REM sleep, the speech is more rational, affective, and coherent, while during non-REM sleep, the speech often seems absurd, and disorganised, especially in stage 3.⁶

Causes of sleep talking

There are many possible causes of sleep talking, including genetics, psychological factors such as stress or anxiety, physiological factors like illness, and in some cases, it can be caused by medication.

Genetic factors

Studies have found that sleep talking can run in families, as genetics are likely to account for over 50% of the reasons why some people talk in their sleep. This is likely since some types of genetic mutations can cause someone to be more likely to talk in their sleep.⁶

Psychological factors

Although there is a lack of evidence of psychological factors being a common reason for sleep talking, there are some suggestions that emotional stress and anxiety may trigger an episode of sleep talking.⁶ Also, sleep talking is more common in patients with diagnosed psychiatric disorders, mainly PTSD (post-traumatic stress disorder).⁷

Dreams

While dreams are not a psychological disorder, they are still an important aspect of psychology, and they do have a relationship with sleep talking. It is believed that sleep talking provides a unique opportunity to understand the purpose of dreams and the different stages of sleep. Some studies found a high correlation between verbal utterances during sleep talking, and the subsequent recall of dreams.⁶ But sleep talking can tell us even more about our brain while we sleep.

Other studies found that the types of words used reflect the emotional state of the brain in a given situation. For example, someone may sleep talk and say negative words, or even swear or be verbally aggressive, which is likely to suggest that the emotional state of this person is not very positive.

Sleep talking also often mimics a conversation-like pattern, as if the sleep talking person were speaking with another person, which can imply that someone is having an imaginary dialogue in their dreams. To summarise, sleep talking is in fact a window into the emotional, and social aspects of dreaming.⁷

Physiological factors

It is important to know that, even when people sleep talk with words and phrases that make sense, often sleep talking consists of mumbling, whispering, or even lip moving without sound. This can be due to the fact that the speech production system of the brain (the Broca’s and Wernicke’s areas) may be partially active, causing changes in articulation and problems with verbal output, but still allowing signs of speech to be present.⁷ It can also be caused by the lack of coordination between brain regions while one is asleep.⁶

Sleep disorders 

Studies have also shown that sleep talking is not an isolated sleep abnormality, but usually occurs alongside other parasomnias, such as non-REM arousal disorders like sleepwalking, sleep terrors, or sleep apnea.⁷

Fever and increased body temperature

Finally, sleep talking can also be caused by common illnesses such as fever, which can trigger an episode of sleep talking in some individuals. Fever, or any illness that causes a high temperature, can cause so-called “fever dreams”, which are often more vivid and ambiguous compared to normal dreams. The reasons for these dreams are unknown; however, it is speculated that they can be caused by “overheating” of the brain, which then interrupts REM sleep and causes changes to regular sleep patterns, which can also be exhibited by sleep talking.⁸

Medication and substances 

The last factor that can potentially cause sleep talking is certain types of medications, such as antidepressants and other psychoactive medications. There are also claims that substances like drugs, alcohol, caffeine, or nicotine can cause sleep talking; however, there is not enough scientific research in these areas to know for sure.^6,7,9

Summary

In summary, sleep talking occurs in all stages of sleep, but can differ between the REM and non-REM stages of sleep. Sleep talking is primarily caused by genetics, but dreams, psychological disorders, fever, and some types of medication can also have an effect. Importantly, it is key to remember that sleep talking in most cases is not harmful, and it is a natural occurrence for everyone from time to time, with some people being more prone to it than others.