Understanding Sleep Cycles and Their Impact on Health

Human sleep is divided into two main categories—REM sleep and non-REM sleep. During REM (rapid eye movement) sleep, the eyes move rapidly, and the thalamus actively sends information throughout the brain; while this type of sleep is essential, it is not considered a restful sleep stage. Conversely, during NREM (non-rapid eye movement) sleep, the brain and body are calm, allowing processes of restoration and relaxation to occur. This type of sleep makes up 75% of nightly rest. Scientists have found that non-REM sleep can be further broken down into three different stages, all of which boast unique benefits and characteristics. A complete sleep cycle takes 90-110 minutes, and a healthy night of sleep should include 4-5 of these cycles.

The transition to sleep occurs much earlier than many people realize. Life stressors, excess stimulation, and environmental factors can keep the brain active even when the body is physically tired, preventing the transition from wakefulness to N1 sleep. However, once drowsiness finally takes over, alpha waves become the predominant pattern and slow the brain's activity, transitioning the body into the first stage of NREM sleep. N1 sleep is the shortest sleep cycle, taking an average of 1-7 minutes. Since this is the lightest stage of sleep, people can be awakened easily without mental fog or confusion.

The second stage of NREM sleep, often abbreviated as "N2," occurs when body temperature drops and the heart rate slows. These physiological changes are accompanied by the presence of sleep spindles and K-complexes which are powerful neuronal firings and long delta waves. This brain activity plays an important role in consolidating memories and is vital to maintaining brain health. N2 sleep lasts approximately 25 minutes but increases in each subsequent cycle.

N3 sleep, colloquially referred to as "deep sleep," offers the most robust health benefits. During this stage of sleep, brain activity is characterized by very slow delta waves, which experts suggest allow the body to heal and grow most effectively. Living up to its "deep sleep" moniker, awakening someone in N3 sleep proves to be difficult in most cases and often results in mental fogginess or impaired mental performance. Unfortunately, recent studies suggest this type of sleep decreases with age, which may account for the slower healing times older adults experience.

REM sleep accounts for almost 25% of nighttime sleep and is easy to recognize—during this stage, the eyes move rapidly back and forth, breathing is irregular, and muscles are atonic. Surprisingly, brain activity during this stage mirrors that of wakefulness, boasting primarily active beta waves. Because of these active waves, many people experience vivid dreams or night terrors during REM sleep. The REM stage typically lasts 10 minutes per cycle, although each subsequent stage lengthens throughout the night.

Circadian rhythms refer to the typical behavior patterns of a 24-hour cycle, and these rhythms have a profound impact on human hormones, eating habits, and body temperatures. In humans, the Period and Cryptochrome genes maintain circadian rhythms and naturally help regulate sleepiness by coding proteins that reside in a cell's nucleus. However, signals from the environment (most often different types of light) can influence when and how these genes turn on or off. When light disrupts the genes and circadian rhythm, many people find it difficult to sleep.

While environmental factors and circadian rhythms play a significant role in sleep regulation, genetics may have the biggest impact when it comes to quality sleep. Many gene mutations affect the natural circadian rhythm, and studies find that several sleep disturbances are directly related to genetic conditions. Some disorders that affect sleep, such as restless leg syndrome and sleep apnea, have familial aggregations that suggest an underlying genetic component.

Sleep changes drastically across the lifespan. Babies take frequent naps and may sleep as much as 17 hours a day. The need for daytime sleep persists through toddlerhood, and elementary-aged children require 10-12 hours of sleep per night. While recommendations for adults have changed over the years, the most recent research suggests that adults should aim for 7-9 hours of sleep per night. Older adults need the same amount of sleep as their younger counterparts but may find it difficult to achieve due to physiological changes.

While many think of insomnia or sleep disturbances as a nighttime problem, many times, the culprit is actually a daytime phenomenon. Lifestyle factors, such as stress, alcohol consumption, caffeine intake, and diet, can drastically affect sleep quality. Caffeine inhibits sleep by blocking adenosine, a naturally occurring sleep chemical found in the body. While this is helpful in the early morning hours, research finds that caffeine should be avoided at least six hours before bedtime.

Sleep disorders, such as insomnia or sleep apnea, wreak havoc on quality sleep. Sleep apnea is a serious medical condition where breathing starts and stops during the sleep cycle, preventing restorative sleep. Many people with sleep apnea report feeling tired even after a full night's sleep, excessive snoring, and difficulty staying asleep as symptoms of their condition.

Sleep deficiency is not only uncomfortable; it is also linked to a plethora of chronic health conditions. Some serious medical complications associated with poor sleep include heart disease, high blood pressure, obesity, and stroke. But that's not all—inadequate sleep has a profound impact on mental health as well. Depression, anxiety, and even suicidal ideation are linked with insomnia and other sleep disorders.

Sleep hygiene, a fun buzzword in the sleep research community, refers to sleep environment and behavior. Experts suggest that incorporating the four pillars of sleep hygiene—regularity, continuity, quality, and quantity—can lead to drastic improvements for those experiencing poor or inadequate sleep. Research finds that from these pillars, keeping a consistent bedtime, engaging in regular exercise and avoiding caffeine were some of the best ways to improve sleep quality.

As with all scientific efforts, sleep research is ever-changing. The Associated Professional Sleep Society hosts an annual meeting that brings together sleep medicine physicians, sleep experts, and circadian scientists to discuss the latest findings in sleep research. Current topics of research include the effect of chronic cell phone use on sleep rhythms in teens and sleep disorders associated with long-Covid.

New technology offers a plethora of opportunities for individuals to monitor their sleeping habits and quality. Sleep trackers record sleep data primarily through accelerometers, devices that measure discreet movements, and biometrics during slumber. Data from the accelerometer then goes into a general algorithm that estimates the quality and duration of sleep. Unfortunately, while these numbers can give a general picture of sleep health, they are not personalized, and results should be taken with a grain of salt.

The importance of quality sleep cannot be overstated when it comes to maintaining physical and mental health. Responsible for memory consolidation, physical restoration, and neural pathway maintenance, a good night's sleep lays the foundation for a productive day. When lifestyle habits or environmental factors creep in, the pillars of good sleep hygiene offer simple solutions to combat insomnia and sleep challenges.