SCIENCE24 - Science of Sleep Part 2: What We Know Today

My last blog, The Science of Sleep Part 1, covered the historical timeline of our understanding of sleep from ancient times through the present.  This blog, The Science of Sleep Part 2 will present the details of what we know about sleep today.

 

After a short introduction, I will cover what happens when you sleep, how sleep is regulated in our bodies, why we need sleep, the ideal duration of sleep, sleep disorders, the effect of diet, the effect of drugs, and finally, tips for getting a good night’s sleep.

I will list my primary sources at the end.

 

Introduction

I include this short introduction as a “snapshot” of our current understanding of some of the key elements of sleep so that the reader has a top-level perspective on what follows.

Sleep is a state of reduced mental and physical activity in which consciousness is altered and certain sensory activity is inhibited.  During sleep, there is a marked decrease in muscle activity and interactions with the surrounding environment.  While sleep differs from wakefulness in terms of the ability to react to stimuli, it still involves active brain patterns, making it more reactive than a coma or disorders of consciousness.

Sleep occurs in repeating periods, during which the body alternates between two distinct modes: rapid eye movement sleep (REM) and non-REM sleep.  Non-REM sleep is characterized by a slow pace of brain activity, breathing, and heart rate.  It's the more restful and restorative phase of sleep, playing a crucial role in tissue repair, building bone and muscle, and strengthening the immune system.  REM sleep is characterized by increased brain activity, and the occurrence of dreams.  REM sleep plays a crucial role in memory consolidation, learning, and emotional processing.  An internal clock regulates sleep-wake cycles. 

Sleep affects almost every system in the body - from the brain, heart, and lungs to metabolism, immune function, mood, and disease resistance.  A chronic lack of sleep, or getting poor quality sleep, increases the risk of health problems.  Without sleep, we can’t form or maintain the brain pathways that let us learn and create new memories.  Lack of sleep makes it harder to concentrate and respond quickly.

Sleep is a critical part of our daily routine - we spend about one-third of our lives doing it. 

 

What Happens When You Sleep

As explained in the introduction, sleep is divided into two broad types: non-rapid eye movement (non-REM or NREM) sleep and rapid eye movement (REM) sleep.  Non-REM sleep has three stages (N1, N2, and N3) that progress from light to deep sleep.  Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. 

Sleep Stages. There are four stages of sleep:  Stage1 (N1), stage2 (N2), stage3 (N3), and stage4 (REM)

These sleep stages are fundamental to how sleep works.

 

Stage 1 non-REM sleep is the changeover from wakefulness to sleep.  During this short period of relatively light sleep, your heartbeat, breathing, and eye movements slow, and your muscles relax with occasional twitches.  Your brain waves begin to slow from their daytime wakefulness patterns.  This stage usually lasts several minutes.

Stage 2 non-REM sleep is a period of light sleep before you enter deeper sleep.  Your heartbeat and breathing slow, and muscles relax even further.  Your body temperature drops and eye movements stop.  Brain wave activity slows but is marked by brief bursts of electrical activity.  This stage last about 10-25 minutes.

Stage 3 non-REM sleep is the deepest non-REM sleep that you need to feel refreshed in the morning.  It occurs in longer periods during the first half of the night, and can last for 20-40 minutes.  Your heartbeat and breathing slow to their lowest levels during sleep.  Your muscles are relaxed and it may be difficult to awaken you.  Brain waves become even slower.   It is believed that deep sleep plays an important role in recuperation of the body as well as effective thinking and memory.

Stage 4 REM sleep first occurs about 90 minutes after falling asleep.  Your eyes move rapidly from side to side behind closed eyelids.  Mixed frequency brain wave activity becomes closer to that seen in wakefulness.  Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels.  Although dreams can happen during any stage, the most intense dreaming takes place during REM sleep.  Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. 

REM sleep typically accounts for about 20-25% of a night's sleep in adults.  As you age, you spend less of your time in REM sleep.  The REM sleep stage is believed to be essential for the brain, enabling key functions like memory and learning.  As the night goes on, it’s normal to spend a greater percentage of time in REM sleep with most of it occurring in the second half of the night. 

Sleep Cycles. It is important to recognize that what happens during sleep is dynamic. Over the course of one night, you progress through multiple sleep cycles, each of which lasts between 70 and 120 minutes, occurring 4-6 times in a good night's sleep, and is composed of the four separate sleep stages.  You spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages.  The first REM period of the night is typically short (around 10 minutes), but it lengthens with each subsequent cycle. 

The structure of a person’s sleep stages and cycles is known as their sleep architecture.

 

Dreaming. Everyone dreams.  You spend about two hours each night dreaming but may not remember most of your dreams.  Its exact purpose isn’t known, but dreaming may help you process your emotions. Events from the day often invade your thoughts during sleep, and people suffering from stress or anxiety are more likely to have frightening dreams.  Dreams can be experienced in all stages of sleep but usually are most vivid in REM sleep. Some people dream in color, while others only recall dreams in black and white.

Waking from Sleeping. When you wake from sleep, your brain transitions from a state of rest to a state of alertness.  This process involves the release of neurotransmitters and hormones that excite the brain, causing it to become more active and aware.  The transition can be gradual or abrupt, depending on factors like sleep stage and wake-up method. 

The optic nerve senses light, triggering the brain to release cortisol, a steroid hormone, produced by the adrenal glands, which prepares the body to wake up.  After waking, especially from deep sleep, you may experience a groggy, disoriented feeling called sleep inertia.  This usually resolves within 30 minutes to an hour. 

Naps.  Naps are short periods of sleep that one might take during the daytime, often in order to get the necessary amount of rest. Napping is often associated with childhood, but around one-third of American adults partake in it daily. The optimal nap duration is around 10–20 minutes, as researchers have proven that it takes at least 30 minutes to enter slow-wave sleep, the deepest period of sleep.  Napping too long and entering the slow wave cycles can make it difficult to awake from the nap and leave one feeling unrested.

The siesta habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep.  Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people.

 

How Sleep is Regulated in our Bodies

Two internal biological mechanisms - circadian rhythms and a natural sleep drive - work together to regulate when you are awake and when you are asleep.  

Circadian Rhythms. Circadian rhythms, your body’s internal biological clock, regulate a range of physical, mental, and behavioral changes over a 24-hour cycle, affecting hormones, digestion, and body temperature.  Circadian rhythms are a key component of the sleep-wake cycle.  They cause you to be sleepy at night and can help you wake up in the morning without an alarm.  Light, especially bright light in the morning, signals the brain to reset your internal clock.  Circadian rhythms do vary among individuals, influenced by genetics, environment, and lifestyle factors.  Some people are naturally "morning larks" while others are "night owls," reflecting variations in their internal biological clocks. 

 

Your body’s internal biological clock, regulates the sleep-wake cycle.

 

Natural Sleep Drive.  Your natural sleep drive (the self-regulating homeostatic system) keeps track of your need for sleep.  Homeostasis refers to a balance between systems in the body.  The homeostatic sleep drive reminds the body to sleep after a certain time and regulates sleep intensity. This sleep drive gets stronger every hour you are awake and causes you to sleep longer and more deeply after a period without sleep.

These two factors directly affect how much your body feels a need for sleep, reflecting your biological clock, the time of day, and how long you’ve been awake.

Other influences on your sleep-wake needs include medical conditions; medications; stress; sleep environment, including exposure to light from electronic devices; age; and what and when you eat and drink.  Perhaps the greatest influence is the exposure to light.   Specialized cells in the retinas of your eyes process light and tell the brain whether it is day or night and can advance or delay our sleep-wake cycle.   Exposure to light can make it difficult to fall asleep and get back to sleep if you wake up during the night.

Night shift workers often have trouble falling asleep when they go to bed, and also have trouble staying awake at work because their natural circadian rhythm and sleep-wake cycle is disrupted.  Jet lag also interferes with a person's circadian rhythms, creating a mismatch between their internal clock and the actual clock.  Native Intuit people living at high north latitudes must endure long, dark winter days by sleeping 14 hours per day, and then must switch to sleeping only six hours per day during the constant light of summer.

Hormones also play an integral role in signaling and regulating sleep-wake states.  Melatonin, which promotes sleep and is naturally produced as light exposure decreases, is one of the best-known hormones related to sleep. 

Artificial lights were introduced to northern latitude native Inuit people, enabling them to adapt to a sleep pattern of eight to nine hours per night throughout the year.  Before this time, their sleep patterns were highly influenced by changes in light depending on the Arctic season.  They endured the long, dark winter days by sleeping 14 hours per day, and then switched to sleeping only six hours per day during the constant light of summer.

 

Why we Need Sleep

Sleep doesn’t have just one biological purpose, but in fact, through its complexity, is an important contributor to the proper functioning of nearly all of the systems of the body.  We need sleep for our physical and mental health.  Sleep allows our bodies and brains to repair and restore themselves, impacting everything from our mood and energy levels to our immune system and cognitive function.  It's a vital process for maintaining overall health and well-being. 

Sleep is an important contributor to the proper functioning of nearly all the systems of the body.

 

Here's a more detailed look:

Sleep reduces energy expenditure, allowing cells to resupply and prepare for the next day's activities. 

While sleeping, our bodies are less active, which aids in healing injuries and repairing damaged cells. 

Sleep strengthens our immune system, making us less susceptible to illnesses. 

Adequate sleep helps regulate blood pressure and reduces the risk of heart disease. 

Sleep plays a role in regulating blood sugar and maintaining a healthy weight. 

Sleep helps the brain process information, consolidate memories, and clear out waste products. 

Getting enough sleep improves our ability to concentrate and think clearly. 

Sleep deprivation can lead to irritability and mood swings, while adequate sleep can help regulate our emotional state. 

Sleep is crucial for consolidating memories and learning new information. 

Sleep helps reduce stress and improve our overall well-being. 

 

The Ideal Duration of Sleep

Human sleep-needs vary by age and amongst individuals; sleep is adequate when there is no daytime sleepiness or dysfunction.

Your need for sleep and your sleep patterns change as you age, but this varies significantly across individuals of the same age.  There is no magic amount of sleep that works for everybody of the same age.  Babies initially sleep as much as 16 to 18 hours per day, which may boost growth and development (especially of the brain).  School-age children and teens on average need about 9.5 hours of sleep per night.  Most adults need 7-9 hours of sleep a night, even older people.  However, older people may have more trouble getting enough sleep and are more likely to take medications that can interfere with sleep.

In general, people are getting less sleep than they need due to longer work hours and the availability of round-the-clock entertainment and other activities. 

Many people feel they can "catch up" on missed sleep during the weekend, but depending on how sleep-deprived they are, sleeping longer on the weekends may not be enough to replace the sleep they've missed.

Recommended hours of sleep per night by age group.

 

Sleep Disorders        

A sleep disorder is a condition that impairs sleep quality, timing, or duration, resulting in interrupted rest.  These problems can manifest in a variety of ways, including difficulties falling and staying asleep, excessive daytime drowsiness, and atypical sleep patterns. Stress, physical ailments, mental health conditions, lifestyle choices, and poor sleeping habits are all potential causes of sleep disorders.   Sleep disorders can significantly impact physical and mental health, leading to issues like fatigue, irritability, difficulty concentrating, and a weakened immune system.  If sleep problems persist, seeking professional help is important to identify the underlying cause and improve sleep quality. 

Major types of sleep disorders.

 

Different forms of sleep disorders include: 

Insomnia: Trouble getting to sleep or staying asleep, which often leaves you feeling tired during the day.   Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10–15% reporting a chronic condition.

Sleep Apnea:  A condition where your breathing stops and starts while you sleep, resulting in restless nights.  

Restless Legs Syndrome: A strong need to move your legs, usually with discomfort, making it hard to sleep.   

Narcolepsy: Episodes of deep sleep that happen unexpectedly during the day, sometimes with sudden sleep attacks. 

Circadian Rhythm Disorders: When your body’s internal clock doesn’t match the outside world, leading to sleep issues (like shift work sleep disorder or jet lag). 

Parasomnias: Unusual behaviors while you sleep, like sleepwalking, experiencing night terrors, or acting out dreams.   There are a variety of causes for sleep disorders.

Medical Conditions: Heart disease, lung disease, nerve disorders, and pain can all contribute to sleep problems. 

Mental Health Conditions: Stress, anxiety, depression, and other mental health issues can disrupt sleep. 

Substance Use: Certain medications, alcohol, and recreational drugs can affect sleep. 

Age: Certain sleep disorders may be more common in children or older adults. 

Lifestyle Factors: Shift work, jet lag, and changes in routines can disrupt the sleep-wake cycle. 

Untreated sleep disorders can lead to serious health consequences, including increased risk of heart disease, stroke, diabetes, and other medical conditions.  Treatments include improving sleep hygiene, such as a regular sleep schedule, a relaxing bedtime routine, and avoiding caffeine and alcohol before bed; medications to help with insomnia or narcolepsy; cognitive behavior therapy to help change negative thoughts and behaviors that contribute to insomnia; CPAP therapy for sleep apnea, and physical therapy for restless legs syndrome. 

 

The Effect of Diet

Dietary intake can significantly impact sleep quality and duration:

Caffeine: A stimulant that can interfere with sleep, even hours after consumption. 

Alcohol: While it might initially induce sleep, alcohol can disrupt sleep later in the night and reduce overall sleep quality. 

High-Fat Foods: May lead to less restful sleep. 

Excess Sugar: Can cause frequent awakenings during the night. 

Bladder Irritants: Certain foods and drinks, like caffeine, alcohol, and carbonated drinks, can increase the urge to urinate, disrupting sleep. 

Stimulants: Nicotine is a stimulant that should be avoided close to bedtime. 

Foods that can aid sleep: Warm milk, tryptophan-rich foods, and melatonin-rich foods like tart cherry juice may help improve sleep quality.  (Tryptophan is an essential amino acid that the body cannot produce on its own and must be obtained through diet.)

Best and worst foods to promote good sleep.

 

The Effect of Drugs

Certain medications, both over-the-counter and prescription, can alter sleep architecture and quality. 

Stimulants:  Weight loss pills, cold and allergy medications, and some antidepressants can have a stimulant effect, potentially interfering with sleep.

Diuretics: May increase urination and disrupt sleep. 

Medications with caffeine: Some over-the-counter medications contain caffeine, which can interfere with sleep. 

Best and worst foods to promote good sleep.

 

Tips for a Good Night’s Sleep

Getting enough sleep is important for your health.  Here are a few tips to improve your sleep:

Set a schedule: go to bed and wake up at the same time each day.

Exercise for at least 30 minutes most days of the week, but not within a few hours of bedtime.

Avoid caffeine and nicotine late in the day and alcoholic drinks before bed.

Relax before bed - try a warm bath, reading, or another relaxing routine.

Create a room for sleep - avoid bright lights and loud sounds, keep the room at a comfortable temperature, and don’t watch TV or use a smartphone or computer in your bedroom.  The color of light significantly impacts sleep due to its effect on the circadian rhythm.  Blue light (typically from electronic devices), especially in the evening, suppresses melatonin production, making it harder to fall asleep and stay asleep.  Red light, on the other hand, has minimal impact on the circadian rhythm and can even promote melatonin production, making it a good choice for nighttime lighting. 

Don’t lie in bed awake: If you can’t get to sleep, do something else, like reading or listening to calming music, until you feel tired. 

See a doctor if you have problems sleeping or if you feel unusually tired during the day.  Most sleep disorders can be treated effectively.

Ten tips for better sleep.

 

Conclusions

Our understanding of sleep is still evolving, with researchers continually uncovering more about its complex functions and the impact of sleep disorders.  While we know that sleep is essential for physical and mental health, there's much still to be learned about the specific mechanisms and why we need it. 

A key focus of research is to understand the risks involved with being chronically sleep deprived and the relationship between sleep and disease.  People who are chronically sleep deprived are more likely to be overweight, have strokes and cardiovascular disease, infections, and certain types of cancer than those who get enough sleep.  Sleep disturbances are common among people with age-related neurological disorders such as Alzheimer’s disease and Parkinson’s disease.  Many mysteries remain about the association between sleep and these health problems. Does the lack of sleep lead to certain disorders, or do certain diseases cause a lack of sleep? 

 

 

"I love sleep.  My life has the tendency to fall apart when I'm awake, you know?" - Ernest Hemingway.

 

 

Sources

My principal sources include “Sleep,” en.wikipedia.org; “Why Sleep Matters: Historical and Cultural Perspectives of Sleep,” and “Science of Sleep: What is Sleep?,” sleep.hms.harvard.edu; “History of Sleep,” news-medical.net; “The Science of Sleep:  Understanding What Happens When You Sleep,” hopkinsmedicine.org; “Brain Basics: Understanding Sleep,” ninds.nih.gov; “How Sleep Works: Understanding the Science of Sleep,” sleepfoundation.org; “How Sleep Works: Understanding the Science of Sleep,” sleepfoundation.org; “Sleep cycles and stages: do you know how it works?” nightingold.com; “Stages of Sleep: What Happens in a Normal Sleep Cycle?,” sleepfoundation.org; “Sleep disorders: What is it & how to fix it?,” hola.health/health-info/mental-health/sleep-disorders-what-is-it-how-to-fix-it; plus, numerous other online sources.  I particularly want to recognize the use of online artificial intelligence ChatGPT summaries on many subjects related to this blog.