The Importance of Sleep

Sleep has a significant impact on nearly all functions of the body. Whether that effect is positive or negative depends heavily on your sleep hygiene, such as the duration, quality, and timing of your rest. During sleep, the mind and body have the opportunity to rest and rejuvenate. This includes repairing muscles and tissues, as well as enhancing cognitive functions, like problem-solving and memory. Getting enough sleep helps regulate the hormones and neurotransmitters that impact our mood. It also strengthens the immune system, enabling us to better defend against infections and reduce inflammation.22 23

Sufficient sleep is just as vital to human survival as food or water due to its critical role in maintaining our physical and mental health. For adults, an average of 7 to 9 hours of sleep per night is recommended.23 Insufficient sleep can potentially lead to irritability, anxiety, depression, and other serious health issues, such as diabetes or cardiovascular disease.23 24

Sleep plays an important role in maintaining your health and well-being. Even though experts say the biological purpose of sleep is still a mystery, we know it has major effects on almost every part of our bodies — the brain, heart, immune system, nervous system, metabolism, lungs, and more. Sleep doesn’t just affect the physical parts of your body, but mental and emotional aspects, too. A lack of good-quality rest can alter how you react, think, learn, concentrate, form memories, and interact with others.23 24

In children and teens, sleep is particularly vital to proper growth and development, as deep sleep triggers the release of certain growth hormones, including those involved with puberty.25

To determine sleep quality, sleep researchers commonly use subjective and objective evaluative measures. Subjective measures include sleep diaries and questionnaires, while objective measures rely on clinical or commercial devices to track a range of physiological variables: heart rate, voluntary and involuntary body movements, respiratory rate, and brainwaves.1

There’s currently no established method for measuring sleep quality, but commonly used metrics include:1

• The time it takes you to fall asleep (sleep onset latency)
• How many times you wake during the night
• How long you spend awake after initially falling asleep
• The efficiency of your sleep (the time spent sleeping while in bed)

Sleep consists of two phases — rapid eye movement (REM) sleep and non-REM sleep.2 While sleeping, we cycle through these two phases every 80 to 100 minutes, usually completing four to six cycles per night and occasionally waking up briefly between cycles.

Non-REM sleep has three stages:2

• Stage 1 is the transition period between wakefulness and sleep
• Stage 2 is the state of being asleep
• Stage 3 (also known as deep sleep or slow-wave sleep) is determined by a particular pattern that appears in measurements of brain activity; we usually spend more time in this stage early in the night

When we are not in one of these three stages, we are experiencing REM sleep, which is typically marked by twitching of the eyes beneath the eyelids, indicating that your brain is active. Several other distinct physiological changes also act as indicators of REM sleep:2

• Our brain activity is similar to that observed during waking hours
• We typically enter a limp state (to prevent us from acting out our dreams)
• Our body’s ability to regulate its temperature is diminished

REM sleep typically occurs late during periods of sleep and is the phase associated with active dreaming. Bouts of REM sleep are initiated and ended in response to the interaction of certain neural circuits within the medulla, midbrain, and hypothalamus. REM sleep is also believed to assist in providing the necessary energy required for bodily maintenance and repair, neural plasticity, and reproductive health.

REM isn’t exclusive to humans, either. Researchers have found evidence that the sleep phase occurs in other mammals and birds, too, with some even making the claim that it may be present in some reptiles (like the Australian bearded dragon).3 While the findings back up claims of REM’s presence in mammals and birds, reptiles are still up for debate.26

The circadian rhythm is a fundamental part of the body’s internal clock and is responsible for the regulation of our sleep-wake cycle. Influenced by shifts between light, dark, and temperature, our circadian rhythm is perpetually maintaining a 24-hour cycle to keep a consistent balance between sleep and wakefulness.4 One of the primary functions of the circadian rhythm is to control the release of melatonin. Stimulated by darkness and inhibited by light, our rising and falling levels of melatonin tell our body when it’s time to sleep and wake up.

Understanding and adjusting to our natural circadian rhythm can help us achieve an optimal balance between rest and wakefulness, develop better sleep habits, and even reduce the risk of sleep disorders.

Mental health

A disrupted circadian rhythm is commonly found in individuals with certain psychiatric disorders, including:4

• Major depressive disorder (MDD)
• Bipolar disorder
• Anxiety
• Schizophrenia
• Post-traumatic stress disorder (PTSD)

The relationship between circadian rhythm disruption and psychopathology, however, is mostly correlational and not yet fully understood. For instance, it’s still unknown whether circadian rhythm disruption predisposes individuals to developing mood disorders, or if mood disorders lead to disruption of circadian rhythms. There’s also the possibility that circadian rhythm disturbances and mental health concerns are both caused by one or more underlying physiological processes. But even if circadian rhythm disruption is not the sole cause of mood disorders, it can still provoke or exacerbate symptoms in people who are predisposed.4

Tension and stress appear to be important factors in predicting sleep quality. In a 2009 study, 1,125 college students, aged 17-24, were asked what one factor they most strongly attributed to poor or disturbed sleep. The most common answers were academic stress (39%) and emotional stress (25%), followed by light or noise (17%), illness or pain (8%), and lastly, co-sleeping (4%).5

Stress can make it more difficult to fall asleep, and it increases the chance of waking up throughout the night. Agitated thoughts about work, relationships, finances, or school cause increased release of stress hormones, mainly cortisol, which contribute to sleep disruptions.27

Experts recommend avoiding caffeine late in the day, doing relaxing activities (like yoga, meditation, or reading), and crafting an optimal sleeping environment. Limiting your use of electronics before sleep is also important, as the blue light emitted from them can disrupt your brain’s melatonin production.27

Eating habits

Multiple connections have also been discovered between our sleeping and eating habits.6 While no clear food-specific patterns have been confirmed, the available research does indicate that certain types of foods can promote sleep in certain situations. These are generally foods that impact the synthesis of serotonin and melatonin, as well as the availability of tryptophan. Eggs, fish, and nuts (particularly pistachios) are high in melatonin.28

The connection between poor sleep and poor diet, however, has been determined more definitively. Outcomes stemming from lack of sleep that can lead to the increased consumption of calories include:6

• Overeating
• Psychological distress
• Greater sensitivity to food reward
• More energy needed to sustain extended wakefulness
• Changes in appetite hormones (ghrelin and leptin)

Caffeine

Caffeine consumption has also been associated with decreased sleep time and quality.7 A 2013 study in the Journal of Clinical Sleep Medicine found that a dose of 400mg of caffeine given right before bedtime, three hours before, and six hours before all had significant impacts on sleep disturbances.

It’s important to note that, while 190mg is what is considered typical of a 16oz cup of coffee (a lot less than what was used in the 2013 study), some researchers found that the caffeine content of coffee from various chains or brands ranged from 58-258mg. And the dose of caffeine in the same amount of coffee from the same location varied greatly across six consecutive days, from 259mg all the way up to 564mg.29

Alcohol

A much clearer association has been found between poor sleep and alcohol consumption. Snoring, obstructive sleep apnea (OSA), and shortened sleep duration can all result from the adverse effects of alcohol on the respiratory system, which include upper airway narrowing, reduction of hypoglossal muscle activity, increased nasal resistance, and reduced ventilatory responses to asphyxia.8 Other contributive effects of alcohol on reduced sleep quality and duration include disruptions to our circadian rhythm, increased physiological arousal, and competition with neuroimmune and neurotransmitter systems due to ethanol metabolization.

Sleep deprivation is correlated with a range of both physical and emotional effects. In spite of preventive measures, acute and chronic sleep deprivation has been shown to induce neurobehavioral changes in humans beyond just feeling sleepy.9 Diminished psychomotor and cognitive abilities have been observed, such as making more frequent mistakes or errors.

Cognition and attention

Vigilant attention, also called sustained attention, refers to our ability to remain focused over time and is necessary for a wide range of tasks requiring cognitive performance. Sustained attention is often impaired by sleep deprivation and can cause you to take longer to complete tasks or react to stimuli.10 25

Some researchers believe that sleep and attention evolved together and work in tandem to regulate one another.32 And some testing methods for determining the relationship between sleep and cognition, such as the psychomotor vigilance test, have helped mitigate variables (like competency at a task) while effectively assessing scenarios where a quick response time is often required (such as driving).10

However, researchers note that even simple tasks require multiple cognitive processes. So, ultimately, more research is required to determine exactly which of the involved processes are most impacted by sleep deprivation.33

Microsleep

In addition to issues with cognition and attention from lack of sleep, you may also experience episodes of microsleep. These episodes are uncontrollable, and you may be unaware they ever even happened. Your eyes can remain open during microsleep, but your brain stops processing information.25 30 31

Not remembering parts of a drive, school lecture, book, movie, and so on could be a sign that you experienced a microsleep. Some symptoms of microsleep include:31

• Frequent or slow blinking
• Difficulty understanding or processing information
• Hypnic jerks (jolting awake suddenly or waking with a start)
• Excessive yawning

Emotions

Studies have shown that sleep deprivation can negatively impact our ability to process and regulate our emotions, including empathy. A 2014 study published in the Journal of Sleep Research found that a single sleepless night can impair a healthy individual’s ability to feel empathy towards others, a key skill necessary for social interaction in both our personal and work lives.11 These findings were consistent with previous research on the relationship between sleep deprivation and impaired cognitive and emotional functioning.

Sleep deprivation also appears to increase emotional reactivity and disrupt outward displays of emotion, leading to impairment of social functioning.12 Overall, the interactions between sleep, emotions, and social functioning are highly complex, and these findings were observed most clearly in neuroimaging studies and measures of subjective emotional intelligence.

Just one night without sleep can also make it difficult to recognize the emotions of others, as indicated by their facial expressions.13 A study that included 29 men and 25 women examined the effects of sleep deprivation on six agreed-upon emotional categories — happiness, surprise, fear, sadness, disgust, and anger. Researchers found that sleep deprivation adversely affected the participants' ability to recognize subtle facial cues for happiness and sadness, arguably the two most important emotions in terms of prosocial interaction and empathy. All the other, more survival-oriented facial cues (like disgust and anger) were found to be much more resilient to sleep loss.13

According to the CDC, a third of U.S. adults are not getting adequate sleep, many due to diagnosed sleep-related disorders. It’s important to understand not only how sleep deprivation can affect our overall health but also to be able to spot the signs of a sleep disorder before our health begins to decline. The chart below details some of the most common sleep disorders.14

Sleep plays a key role in our overall health, and it is anything but a passive activity. On any given night, we go through four to six sleep cycles (20-25% of which is REM sleep) and dream for around two hours. Our body temperature drops by 1-2 degrees Fahrenheit while our metabolism decreases by around 15%. Not everyone, however, needs the same amount of sleep, is getting the necessary amount of sleep, or is having the same sleep experience.

How many hours of sleep do you need?

According to the CDC, The American Academy of Sleep Medicine, and the Sleep Research Society, the following list details how much sleep is needed per age group:15

• Infants (4-12 months): 12-16 hours of sleep per 24 hours (including naps)
• Toddlers (1-2 years): 11-14 hours per 24 hours (including naps)
• Pre-school (3-5 years): 10-13 hours per 24 hours (including naps)
• School-age (6-12 years): 9-12 hours per 24 hours
• Teenagers (13-18 years): 8-10 hours per 24 hours
• Adults (18-60+ years): 7 or more hours per night

Sleep and depression

Links have been discovered between poor sleep quality and depressive symptoms. According to the National Sleep Foundation's 2023 Sleep in America Poll, the following statistics were found:16

• Adults who slept less than 7 hours per night on weekdays were three times as likely to experience moderate to severe depressive symptoms as those who slept the recommended amount.
• Nearly 1 in 5 adults (19%) who habitually sleep less than 7 hours per night meet probable criteria for a depression diagnosis.
• Nearly 7 out of 10 people (65%) who were dissatisfied with their sleep also experienced mild or greater levels of depressive symptoms, with 31% reporting moderate-severe symptoms.
• A quarter of adults (25%) who were dissatisfied with their sleep also met probable criteria for a depression diagnosis.
• Nearly 2 in 5 people who have difficulty staying asleep (38%) just two nights a week also reported mild or greater levels of depressive symptoms, with 10% saying their symptoms were moderate to severe.
• About 20% of individuals who experience difficulty staying asleep for two or more nights also met probable criteria for a depression diagnosis.

Sleep and sex differences

Evidence shows that men and women experience sleep differently. However, the reasons why remain undetermined, possibly due to the underrepresentation of women and female animals in studies.17 The following list details some of these differences:17

• Women have a 40% greater risk of developing insomnia throughout their lifetime than men.
• One of the most common symptoms of menopause, reported by 33-51% of women, is complaints about sleep quality.
• According to the Food and Drug Administration (FDA), women should only take half the recommended dose of the commonly prescribed sleep-aid Ambien (zolpidem).
• Lower testosterone levels have been associated with decreased sleep efficiency, increased nocturnal awakenings, and less time in slow-wave sleep in men aged 65 years and older.
• Studies suggest that testosterone is linked to a worsening of obstructive sleep apnea in men.

Pregnancy and sleep disturbances

Reports of changing sleep habits, such as insomnia, are widespread among pregnant individuals. A 2012 study delved further into the most common complaints:18

• Nearly three-quarters of pregnant women (71%) said their sleep habits changed during pregnancy.
• 10.1% of pregnant women had or developed a snoring habit.
• 61.7% of pregnant women said they began to sleep earlier than before.
• Over half of pregnant women (51.2%) reported experiencing symptoms of insomnia.
• Reasons for insomnia-like behaviors included frequent visits to the toilet (39.3%), inability to find a comfortable position (30.7%), and restless legs (13.7%).
• Being in the third trimester, being 20 years old or older, and having a score of 18+ on the Beck Depression Inventory (BDI) were all found to be prominent risk factors for insomnia.

Sleep aids

Many people use sleep aids (such as melatonin) to try and overcome sleep disturbances, but research has found that the lasting effects of these aids can lead to additional concerns:

• Around 8 in 10 adults (8.2%) reportedly take medication to help them fall or stay asleep four or more times a week (6.6% of men and 9.7% of women).19
• The percentage of men and women who reportedly take medication for sleep four or more times a week increased with age and was more common in women across all age groups.19
• 80% of sleep aid users report feeling some residual effects, including drowsiness, grogginess, sluggishness, difficulty concentrating and remembering, and sleeping too much.20
• Individuals with anxiety, depression, schizophrenia, or fibromyalgia were more likely to experience residual symptoms.20
• Comorbid conditions — such as congestive heart failure and psychiatric disorders — can be potentially aggravated or exacerbated by the side effects of sleep medications.20
• Out of 31 over-the-counter melatonin supplements, actual melatonin concentration ranged from 83% below to 478% above the listed dosage; 70% of the tested products were found to contain a concentration of melatonin less than or equal to 10% of what was indicated.21

Because sleep is as essential to our survival as food or water, most adults should be getting an average of 7-9 hours of sleep per night. Along with stress, alcohol, and caffeine, circadian rhythm disruptions are a common cause of inadequate sleep. Influenced by shifts between light, dark, and temperature, the circadian rhythm is responsible for maintaining our 24-hour sleep-wake cycle. This rhythm keeps us in a consistent balance between periods of sleep and wakefulness, helps us develop better sleep habits, and may even prevent sleep disorders that can negatively affect our physical and mental health.

Millions of people do not get adequate sleep due to social, economic, and behavioral factors, as well as diagnosed sleep-related disorders. Not getting enough sleep can lead to a range of emotional problems, including irritability, anxiety, and depression.

Sleep deprivation is also correlated with diminished cognition and impaired social functioning, which can negatively impact our ability to process emotions — it can even affect our ability to recognize the subtle facial cues of happiness and sadness.

Understanding sleep and its impacts on our bodies is the first essential step in preventing or treating the many sleep disorders or disturbances we may experience throughout our lives.