Understanding Sleep Stages and Their Impact on Health

Sleep is a complex and vital process consisting of different stages, each with unique characteristics and functions. Recognizing the significance of these stages is essential for maintaining overall well-being. 

Think of sleep as a time when your brain and body receive essential updates. Just as your computer or phone needs updates to function efficiently and stay secure, your body needs sleep to repair cells, consolidate memories, and regulate hormones.

Essentially, sleep is characterized between non-rapid eye movement sleep (non-REM) and rapid eye movement sleep (REM) sleep. 

Impact of Sleep Stages on Health

Each sleep stage contributes to different aspects of physical, cognitive, and emotional health. Adequate time spent in each stage is vital for:

• Physical Health: Proper sleep stages promote tissue repair, immune function, and physiological restoration. Randomized control trials (RCTs) have demonstrated that sleep deprivation impairs immune response, increasing susceptibility to infections (Irwin et al., 1994).
• Mental Health: Memory consolidation and learning are significantly enhanced during REM and NREM sleep. Meta-analyses indicate that sleep plays a critical role in memory processes, particularly in stabilizing and integrating memories (Stickgold, 2005).
  • Adequate REM sleep helps in regulating emotions and reducing stress. Studies show that sleep disruptions are linked to increased emotional instability and psychiatric disorders (Harvey, 2008).

Disruptions in sleep stages can lead to various health issues such as decreased cognition, impaired memory, poor coordination, mood fluctuations, increased risk of heart disease and diabetes, weight gain, and compromised immune function. Therefore, prioritizing healthy sleep habits and ensuring sufficient time for each sleep stage is crucial for overall health and well-being.

What is Non-REM Sleep?

Non-REM (non-rapid eye movement) sleep is a crucial sleep stage divided into stages 2 and 3-4 (N3) in humans. It is essential for physical and intellectual performance, involving synchronized brain activity with slow waves and spindles. Non-REM sleep helps normalize synapses, ensuring cellular homeostasis and reducing synaptic connections. This sleep stage is vital for energy conservation and memory processing, contributing significantly to overall sleep quality and health ¹.

Non-REM (NREM) sleep is divided into three stages: N1, N2, and N3.

1. N1 (Stage 1)
   • This is the lightest stage of sleep, serving as the transition between wakefulness and sleep. It involves light sleep where one can be easily awakened.
   • Functions: The brain begins to produce theta waves, and there is a gradual decrease in muscle activity. This stage is critical for initiating sleep and can last for 1-7 minutes in a full sleep cycle.
2. N2 (Stage 2)
   • This stage represents a deeper level of sleep compared to N1, accounting for about 45-55% of total sleep in adults. This can last between 30-60 minutes typically in a sleep cycle.
   • Functions: During N2, heart rate and body temperature drop, muscles also relax.  Sleep spindles and K-complexes ² which are specific types of brain wave signatures, occur and are believed to play a role in memory consolidation and the brain preparing to enter N3 or deepest stage of sleep maintenance.
3. N3 (Stage 3)
   • Known as slow-wave or delta sleep, this is the deepest stage of sleep and accounts for approximately 15-25% of total sleep in adults.
   • Functions: Significant physiological processes such as tissue repair, toxin removal,  growth, and cell regeneration occur. The immune system is strengthened during this stage, which is crucial for recovery and overall health. The body is also the least metabolically active.

Slow waves and spindles are essential components of non-rapid eye movement (NREM) sleep:

• Slow waves: These are ∼1 Hz, high-amplitude oscillations primarily generated in the cortex. They play a crucial role in memory, cognition, and neuronal plasticity.
• Spindles: These are 12-16 Hz oscillations initiated in the thalamus and regulated by thalamo-cortical circuits. Spindles are involved in memory, learning, and the restorative aspects of sleep.

Both slow waves and spindles ³ are vital for brain function and overall health during NREM sleep.

Impact of Non-REM Sleep on Health

NREM sleep is vital for:

• Restoration and Repair:
  • NREM sleep is associated with physical restoration and repair processes in the body. It promotes muscle recovery, growth, and organ- and tissue-level repair, as well as the release of growth hormones (Kushida, 2004).
• Memory Consolidation:
  • NREM sleep is crucial for memory consolidation and learning. It helps to evaluate which information is worth retaining, solidifying and integrating different bits of information and experiences together into long-term memory. (Rasch and Born, 2013)
• Immune Function:
  • NREM sleep is involved in immune system regulation. It supports immune function and helps the body fight off infections and diseases, Besedovsky et al. (2012)
• Glucose Regulation:
  • NREM sleep plays a role in maintaining normal glucose homeostasis. Disruptions in NREM sleep, such as reduced slow-wave sleep (SWS), have been associated with decreased insulin sensitivity and increased risk of type 2 diabetes, Spiegel et al. (2005).
• Background organ-level communication and Regulation:
  • NREM sleep helps regulate autonomic functions such as heart rate, blood pressure, and body temperature. It is characterized by parasympathetic dominance, promoting relaxation and restoration (Somers et al., 1993).
  • NREM sleep can affect the activity of respiratory muscles. It may lead to changes in the activity of inspiratory muscles, such as the diaphragm and intercostal muscles. (Eckert et al. 2010)

NREM Sleep and Glucose Regulation

Understanding Basal Metabolic Rate (BMR) is crucial for health as it represents the energy expended by the body at rest to maintain vital functions. It’s noteworthy that during sleep, the metabolic rate drops by only about 15%, indicating the significant physiological activities occurring even while we rest.

NREM sleep plays a significant role in glucose regulation in the body. Here are some key impacts of NREM sleep on glucose regulation:

• Glucose Metabolism:
  • During NREM sleep, there is a reduction in cerebral glucose utilisation ⁹, which allows for the conservation of glucose and promotes energy conservation in the brain.
• Insulin Sensitivity:
  • NREM sleep has been associated with improved insulin sensitivity. Studies have shown that selective suppression of deep NREM sleep, also known as slow-wave sleep (SWS), leads to marked decreases in insulin sensitivity and reduced glucose tolerance. (Tasali et al. 2008)
• Glucose Homeostasis:
  • NREM sleep is involved in maintaining normal glucose homeostasis. Disruptions in NREM sleep, such as sleep apnea or sleep deprivation, can impair glucose regulation and increase the risk of insulin resistance and type 2 diabetes, Punjabi (2008)
• Sleep Apnea and Hypoxemia:
  • Sleep apnea, a condition characterized by interrupted breathing during sleep, can lead to hypoxemia (low oxygen levels) and negatively impact glucose regulation during NREM sleep. Severe nocturnal hypoxemia in patients with sleep apnea has been associated with altered glucose variability during NREM sleep, Stamatakis and Punjabi (2010).

REM Sleep

Rapid Eye Movement (REM) sleep is a distinct sleep stage characterized by vivid dreams, desynchronized brain activity, and muscle paralysis. REM sleep plays crucial roles in-memory processing, emotional regulation, and overall brain health. Brain regions like the pons and medulla regulate REM sleep, involving neurotransmitters like acetylcholine and GABA. Understanding REM sleep is essential for treating sleep disorders and neurological conditions ^4,5,6

Characteristics: REM sleep accounts for approximately 20-25% of total sleep in adults. It is marked by increased brain activity, similar to wakefulness, and is divided into tonic and phasic phases. The tonic phase involves low muscle tone, while the phasic phase includes bursts of rapid eye movements.

Tonic and phasic phases during sleep refer to distinct microstates within Rapid Eye Movement (REM) sleep ^7,8.

• Tonic REM: Characterized by sustained muscle activity, it shows minimal eye movements and is associated with decreased sensory processing.
• Phasic REM: Involves rapid eye movements, muscle atonia, and intense cortical activity. Phasic REM exhibits heightened sensory processing and distinct cortical oscillations.

Functions: REM sleep is crucial for several brain functions:

• Memory Consolidation: Involves the processing and consolidation of memories. Meta-analyses have shown that REM sleep facilitates the consolidation of procedural and emotional memories (Rasch & Born, 2013).
• Emotional Regulation: Helps to regulate and integrate emotions, contributing to overall emotional well-being. Studies indicate that REM sleep decreases emotional reactivity and helps in processing emotional experiences (Walker & van der Helm, 2009).
• Brain Development: Essential for brain development in infants and children, promoting neural maturation and the formation of neural connections (Mirmiran et al., 2003).
• Learning and Creativity: Enhances problem-solving abilities and creativity. Research shows that REM sleep facilitates the integration of uncorrelated information, fostering creative problem-solving (Cai et al., 2009).

Functions of REM Sleep

REM sleep serves several important functions in recovery, memory, emotions and cognitive development:

• Memory Consolidation:
  • Studies: REM sleep is involved in the processing and consolidation of memories. A meta-analysis by Diekelmann and Born (2010) highlighted the role of REM sleep in the consolidation of emotional and procedural memories.
• Emotional Regulation:
  • Studies: REM sleep is associated with emotional regulation and processing. It helps to regulate and integrate emotions, contributing to overall emotional well-being. Research by Walker and van der Helm (2009) shows that REM sleep reduces emotional reactivity and helps process emotional experiences.
• Brain Development:
  • Studies: REM sleep is essential for brain development, particularly in infants and children. It promotes neural maturation and the formation of neural connections. Mirmiran et al. (2003) discuss the importance of REM sleep for the developing brain.
• Learning and Creativity:
  • Studies: REM sleep enhances problem-solving abilities, creativity, and the ability to think abstractly. Cai et al. (2009) found that REM sleep facilitates creative problem-solving by integrating uncorrelated information.
• Restoration and Recovery:
  • Studies: While REM sleep is not primarily focused on physical restoration like NREM sleep, it still plays a role in overall restoration and recovery. Studies show that REM sleep is involved in energy replenishment, tissue repair, and immune function (Vyazovskiy & Harris, 2013).
• Regulation of Arousal:
  • Studies: REM sleep helps regulate the sleep-wake cycle and arousal levels. It is involved in the transition between sleep and wakefulness. Hobson et al. (1975) discuss the role of REM sleep in maintaining homeostasis and regulating arousal.
• Dreaming:
  • Studies: REM sleep is strongly associated with dreaming. It is the stage of sleep where vivid and often bizarre dreams occur. Research by Revonsuo (2000) suggests that dreaming during REM sleep may serve an evolutionary function for simulating threatening events and practicing avoidance behaviors

Symptoms of REM Sleep Behavior Disorder (RBD)

Symptoms include:

• Acting out dreams:
  • Studies: During an episode of RBD, individuals physically act out their dreams. This can involve movements such as kicking, punching, flailing, grabbing, or jumping. Research by Schenck et al. (2013) discusses the clinical features of RBD.
• Vocalizations:
  • Studies: People with RBD may make noises such as talking, laughing, shouting, emotional outcries, or even cursing during sleep. Research by McCarter et al. (2012) discusses vocal manifestations in RBD.
• Dream recall:
  • Studies: Upon awakening, individuals with RBD can often remember the details of their dreams, which match the behaviors they acted out during sleep. Research by Olson et al. (2000) discusses dream enactment in RBD.
• Violent or action-filled dreams:
  • Studies: RBD episodes are often associated with vivid, intense, action-filled, and sometimes violent dreams. Research by Vetrugno et al. (2009) discusses the nature of dreams in RBD.
• Timing of episodes:
  • Studies: RBD episodes typically occur at least 90 minutes after falling asleep, usually during the later portions of sleep. Research by Boeve et al. (2007) discusses the timing of RBD episodes.

Medications Impacting REM Sleep

Certain medications can affect REM sleep, such as:

• Antidepressants:
  • Studies: Many antidepressants, particularly those that increase serotonin function, can suppress REM sleep. This includes selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine and serotonin-norepinephrine reuptake inhibitors (SNRIs) like venlafaxine. A review by Wilson and Argyropoulos (2005) discusses the impact of antidepressants on sleep architecture.
• Antipsychotics:
  • Studies: Some antipsychotic medications, such as haloperidol, can either augment or inhibit REM sleep, depending on the individual. Research by Monti and Monti (2004) discusses the effects of antipsychotics on sleep.
• Antiseizure Medications:
  • Studies: Certain antiseizure medications, including sodium channel blockers and GABA enhancers, can affect REM sleep. Research by Bazil (2003) discusses the impact of antiseizure medications on sleep.
• Benzodiazepines:
  • Studies: Benzodiazepines, such as diazepam and lorazepam, can suppress REM sleep. Research by Kales et al. (1983) discusses the effects of benzodiazepines on sleep stages.
• Acetylcholinesterase Inhibitors:
  • Studies: Acetylcholinesterase inhibitors, like rivastigmine, which are used in the treatment of Alzheimer’s disease, have been found to reduce REM latency. Research by Suh et al. (2004) discusses the effects of acetylcholinesterase inhibitors on sleep.

Improving Sleep Quality

Strategies to improve sleep include:

• Stick to a consistent sleep schedule:
  • Go to bed and wake up at the same time every day, even on weekends. This helps regulate your body’s internal clock and promotes healthy sleep patterns (Borbély & Achermann, 1999).
• Create a relaxing bedtime routine:
  • Engage in calming activities before bed, such as taking a warm bath, listening to soothing music, or practicing relaxation techniques like deep breathing or meditation. This can help signal to your body that it’s time to wind down and prepare for sleep (Riemann et al., 2010).
• Avoid stimulants and heavy meals before bedtime:
  • Limit your intake of caffeine and nicotine, as they can interfere with sleep. Additionally, avoid consuming large meals or spicy foods close to bedtime, as they can cause discomfort and disrupt sleep (Drake et al., 2013).
• Exercise regularly:
  • Engaging in regular physical activity during the day can promote better sleep, including REM sleep. Aim for at least 20 to 30 minutes of moderate exercise most days of the week, but try to finish your workout several hours before bedtime to allow your body to wind down (Driver & Taylor, 2000).
• Create a sleep-friendly environment:
  • Make sure your bedroom is cool, dark, and quiet. Use curtains or blinds to block out external light, consider using earplugs or a white noise machine to mask disruptive sounds, and ensure your mattress and pillows are comfortable and supportive (Buysse et al., 2008).
• Manage stress:
  • High levels of stress can interfere with sleep, including REM sleep. Practice stress management techniques such as mindfulness, journaling, or talking to a therapist to help reduce stress and promote better sleep (Morin et al., 2006).

Overall, both REM and NREM sleep stages are critical for maintaining optimal health. Understanding their functions and the impact of disruptions can help in developing better sleep habits and improving overall well-being.

Sources:

1. https://pubmed.ncbi.nlm.nih.gov/22823399/
2. https://pubmed.ncbi.nlm.nih.gov/31753592/
3. https://www.ncbi.nlm.nih.gov/books/NBK557469/
4. https://pubmed.ncbi.nlm.nih.gov/27060683/
5. https://pubmed.ncbi.nlm.nih.gov/29846796/
6. https://pubmed.ncbi.nlm.nih.gov/17884926/
7. https://pubmed.ncbi.nlm.nih.gov/31377324/
8. https://pubmed.ncbi.nlm.nih.gov/23853561/
9. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2929498/