The Criticality of Sleep

Allison Sayre, MSN, WHNP-BC

Sleep is not a luxury; it is an essential. A healthy sleep schedule forms the foundational underpinning of wellbeing, resilience, and longevity. From cellular repair to emotional regulation, the benefits of sleep transcend over every organ system. Yet, in today’s world, we tend to glorify busyness and workaholism, which causes us to overlook and undervalue the importance of sleep and rest.

The effects of lack of sleep include a wide range of health consequences, including impaired immune function, cognitive decline, and emotional instability, just to name a few. Sleep is essential for both physical and mental well-being, as it plays a key role in modulating metabolic, cardiovascular, immune, and neuropsychiatric health. This is particularly concerning, given that an estimated 50 to 70 million Americans are affected by chronic sleep disorders. [1]

In this article, we review some of the changes that happen in your brain and the rest of your body during sleep. Our review focuses on compelling clinical data that investigates several of the consequences of sleep deprivation, and why it is biologically imperative that we prioritize healthy sleep habits.

The Stages of Sleep

Sleep can be broadly divided into Rapid Eye Movement (REM) sleep and non-REM sleep (NREM). NREM can then be further divided into 3 separate stages. An 8-hour night of sleep is made up of approximately 5 of these REM/NREM cycles, lasting about 90 minutes each. While REM is controlled predominantly by the circadian rhythm, NREM tends to be under homeostatic control. Each stage of sleep can be differentiated by the differences in the frequency and amplitude of brain wave activity. [2]

1.  Stage 1: (~10% total sleep time) This is light sleep where you are drifting off, but can wake easily. Muscle activity, respiration, and heart rate slow, and there may be occasional muscle twitching. The brain is still producing alpha waves of wakefulness but begins to produce high amplitude theta waves (4-7 Hz), which are lower in frequency. [3]
2. Stage 2: (~50% total sleep time) This stage features a further decrease in heart rate and body temperature. Sleep spindles (brief bursts of rapid, rhythmic brain activity) and K-complexes (large waves that react to external stimuli) occur. These brain activities help to block out disturbances and play a role in learning and memory. [3]
3. Stage 3: (~20% total sleep time) Also known as deep or slow-wave sleep, this stage is essential for physical restoration and growth. Heart rate and respiration slow dramatically, and a person in this stage is much more difficult to wake. Delta waves (0.5-4 Hz), which are slow and high in amplitude, dominate this stage in healthy sleep, and are associated with feeling refreshed and well-rested in the morning. [3]
4. REM Sleep: (~20% total sleep time) Rapid eye movements occur during this stage, along with increased brain activity that resembles wakefulness. It is also the stage of sleep associated with vivid dreaming.

The brain exhibits low-amplitude, mixed-frequency waves like those seen when awake, but the muscle systems are paralyzed, except for those needed for circulation and respiration. When someone is deprived of REM sleep and then allowed to sleep without disturbance, they will typically spend more time in REM sleep to recoup the lost time in REM. This is known as the “REM rebound” and suggests that REM sleep is also homeostatically regulated. [3]

The Science of Sleep: From Brain to Body 

1. Cognitive Function & Mood

When it comes to sleep, both quantity and quality need to be considered. It is not just about how long you sleep, but whether your body cycles through the essential stages. This is especially true for deep sleep and REM sleep, which are crucial for restoration, cognitive function, and emotional regulation. Therefore poor-quality sleep, even if long in duration, can leave you feeling tired and groggy. 
Neuroimaging shows that even just one night of sleep deprivation can lead to significant disruptions to functional connectivity in the amygdala, hippocampus, thalamus, and insula, leading to deficiencies in attention, disrupted working memory, and impaired emotional processing. [4] One night of total lost sleep, or partial sleep over a few nights, negatively impacts clearing of brain waste, brain development, and work or school performance. [5]

When sleep is deprived over long-term periods, data consistently shows the most significant changes occur in the right thalamus of the brain. The functions of the right thalamus include thermoregulation, action, and pain perception. Other common alterations after chronically poor sleep are found in the right-side subgenual anterior cingulate cortex (sgACC), which is associated with reward, reasoning, and gustation/taste. Alterations can also take place in the right-side hippocampus and amygdala, which are associated with negative emotion processing, memory, and olfaction/smell. [5] Together, these findings suggest an association between sleep disturbances, brain asymmetry, and sensory, cognitive, and emotional dysfunction.

2. Metabolic and Cardiovascular Health

Sleep insufficiency has also been shown to disrupt glucose metabolism, insulin sensitivity, and appetite regulation. In a randomized, 2-condition crossover study, 12 healthy men slept either 4 or 8 hours for 2 nights. Subjects who slept less were hungrier before breakfast and dinner and ate more calories than those who slept 8 hours. This was likely due to increases in ghrelin (hunger signals) and decreases in leptin (satiation signals) measured after sleep deprivation. Studies like this help to uncover the mechanisms between sleep deprivation and unhealthy body mass. [6]

Multiple population studies have reported that sleep durations of less than six hours or more than nine hours, are associated with impaired glucose tolerance and downstream metabolic consequences. While this data may point to “too much sleep” being as dangerous to metabolism as sleep deprivation, the long-sleep association is more likely a demonstration of poor quality sleep, dysregulated cytokine responses, and/or existing mood conditions, which are linked to both longer sleep duration and metabolic issues. [7] This hypothesis is supported by one study in particular, in which two nights of 4-hour restricted sleep resulted in participants waking with higher blood glucose and lower blood insulin than those who had their sleep extended to 10 hours. [8]

Additionally, metabolism and cardiovascular health are tightly associated, and therefore sleep may influence both. Epidemiological studies show that chronic sleep deprivation is associated with dysregulation of blood pressure, heart rate, insulin sensitivity, autonomic nervous system activity, and salt and fluid homeostasis. All of which are indicators of cardiovascular risk.

Furthermore, approximately half of all obstructive sleep apnea (OSA) diagnoses are accompanied by dysregulated blood pressure, and at least 30% of patients with dysregulated blood pressure also have OSA. Observational studies suggest that a return to normal sleep for those with OSA may also reduce their risk for cardiovascular health problems. [1]

3. Immune Resilience & Longevity

Sleep and the immune system have been shown to be closely connected, each influencing the other. An activated immune system can affect the quantity, quality, and restoration provided by sleep, and poor sleep can hamper both the innate and adaptive immune defenses. Whether only a few hours of sleep restriction, or chronic sleep disturbances, it is well-documented that white blood cell counts are reduced when sleep is reduced. More specifically, monocytes and lymphocytes (including B-cells, T-cells, and NK cells) are the most impacted by loss of sleep. [2]

Additionally, like the metabolism data, both too little and too much sleep has been linked with shorter lifespan. Those consistently sleeping 7 to 8 hours per night are likely to live a longer life than individuals sleeping less than 6 or more than 9 hours. Moreover, people who sleep between seven and eight hours have an 18.1% higher likelihood of “healthy aging”, which was defined in this study as “remaining free of major chronic diseases, with good physical function, cognitive function, mental health, and active social participation”. It should be noted that this study also emphasized the importance of both consistency and quality of sleep. Sleeping less and then more to average 7-8 hours does not provide the same health benefits of sleep as routine, adequate sleep nightly. [9]

In Summary

Sleep is not simply a period of rest, rather an active and dynamic process that plays a central role in nearly every aspect of human health. From repairing tissues and consolidating memories, to supporting mood and immune defenses, adequate sleep is essential for physical, cognitive, and emotional resilience. As the research continues to demonstrate, prioritizing both the quality and consistency of sleep is one of the most impactful steps individuals can take to support long-term well-being and disease prevention.

Disclaimer:
The information provided is for educational purposes only. Consult your physician or healthcare practitioner if you have specific questions before instituting any changes in your daily lifestyle including changes in diet, exercise, and supplement use.

Allison Sayre, MSN, WHNP specializes in women's health and functional medicine, blending both traditional and integrative approaches. With over 18 years of experience, she has empowered women to reclaim their health through personalized nutrition and supplementation, hormone balancing, and lifestyle modifications. She received her Bachelor of Science from Mount Carmel College of Nursing and her Master of Science from the University of Cincinnati. She has been a certified women’s health nurse practitioner since 2014 and has continued her education and training in functional medicine from both the Institute for Functional Medicine as well as the American Academy of Anti-Aging Medicine.