Quality sleep is important for many aspects of health, yet many of us struggle to get enough sleep every night. This post discusses the proven health benefits of sleep.
What is Sleep?
Sleep is defined as a natural and reversible state of reduced responsiveness and relative inactivity, accompanied by a loss of consciousness .
Sleep occurs in regular intervals and humans have a consistent need for sleep, so a loss or delay in regular sleep results in subsequently prolonged sleep .
There are two types of sleep, i.e. non-rapid-eye-movement (NREM) sleep and rapid-eye-movement (REM) sleep.
Rapid Eye Movement (REM) Sleep:
REM sleep is characterized by high-frequency low-voltage brain electrical activity (as measured by an electroencephalogram (EEG)) and bursts of rapid contractions of eye muscles, causing the eyes to move rapidly.
A healthy young person’s normal night of sleep typically includes 4-5 distinct REM cycles. REM sleep accounts for 20% of the total time spent asleep .
Non-Rapid Eye Movement (NREM) Sleep:
Most of a night’s sleep is spent in NREM sleep, which is further divided into 4 sub-stages (stages I-IV).
Stage I (N1) is the lightest stage of sleep
Stage II (N2) sleep is defined by the emergence of specific types of peaks in the EEG spectrum known as K-complexes and sleep spindles. N2 sleep typically accounts for more than half of one night’s sleep.
Stage III and IV (N3 and N4) are the deepest stages of sleep and are characterized by slow brain waves called delta brain waves. Together stages III and IV are referred to as “Slow-wave sleep” (SWS).
Slow wave sleep (SWS) is considered the most restorative stage of sleep where the greatest impact of immune regulation happens.
Throughout one period of sleep, both REM and NREM sleep alternate cyclically. For a healthy young person, the first progression through the four non-REM sleep stages typically takes 70-100 minutes.
Both REM and NREM sleep are important. Irregular cycling and/or the absence of sleep stages are associated with sleep disorders .
Typically people with inflammation tend to have trouble getting enough deep sleep (stages 3 and 4) because inflammation can result in HPA axis activation and CRH increase, which reduces deep sleep .
Sleep and Brain Health
1) Improves Memory and Cognitive Functions
Sleep is essential for effective cognitive function and sleep improves both cognitive function and memory, while a lack of sleep is detrimental to cognitive function .
- Sleep helps strengthen and stabilize new memories acquired before sleep [7, 8, 9].
- Sleep promotes the reprocessing of fresh memories and their integration into the pre-existing network of long-term memories [10, 11, 12].
- Sleep loss diminishes many cognitive abilities including attention, language, reasoning, decision-making, learning and memory [13, 14, 15].
- In children, shortened sleep duration, especially before the age of 41 months, is associated with lower cognitive performance and developmental tests .
- Humans completely deprived of sleep showed deficits in the ability to perform unconscious and repeated movement memory such as bicycle riding and shoelace tying as well as deficits in short-term memory .
- A short period of sleep prior to learning can enhance the capacity to remember new information .
- Naps can reduce sleepiness and improve cognitive performance .
2) Helps Remove Waste from the Brain
The glymphatic system clears metabolic waste from the mammalian brain while the fluid surrounding the brain and spinal cord cells (cerebrospinal fluid) removes beta-amyloid metabolites from the brain [20, 21].
Sleep stimulates the glymphatic system and the cerebrospinal fluid to clear metabolic waste from the adult brain .
When individuals sleep well, the glymphatic system can effectively remove cellular waste byproducts that have accumulated outside and inside of brain cells .
3) Sleep as a Neuronal Detoxification process
Levels of oxidized glutathione, a sleep-inducing substance, increase at the end of the day and during sleep. These high levels of oxidized glutathione may counteract glutamate toxicity and prevent neuronal cell death level .
4) Sleep and Mental Illness
Most patients with depressive disorders also have sleep disturbances. Difficulty in initiating or maintaining sleep or both have been reported in about three-quarters of all depressed patients .
Patients with persistent insomnia have a 2 to 3.5-fold increased risks of developing depression than those without insomnia [26, 27]. 72.7% of youths with major depression also reported insomnia and hypersomnia .
Sleep disturbances are also commonly associated with other psychiatric illnesses, including 
- generalized anxiety disorder
- panic disorder
- posttraumatic stress disorder
Unfortunately, most antidepressant medications suppress rapid eye movement (REM) sleep .
32% of people with brain fog also have a diagnosed sleep disorder, most commonly insomnia, sleep apnea or restless leg syndrome .
In mental health practice settings, children and adolescents with ADHD frequently reported sleep problems, particularly difficulty in initiating and maintaining sleep .
Sleep is Important for Healthy Metabolism
5) Reduces Appetite
The two hormones that control appetite regulation are leptin and ghrelin. Leptin decreases your appetite, while ghrelin increases it.
In sleep-deprived young men, leptin levels decreased by 18% and ghrelin levels increased by 28% causing a total increase in appetite of 23% when compared to the levels present after healthy sleep .
6) May Help with Weight Loss
Unhealthy sleep is associated with obesity and eating problems .
Additionally, a National Health and Nutrition Examination Survey analysis showed that adults who slept less than 7 hours per night were more likely to be obese .
People who slept only 5.5 hours per night lost 55% less body fat and 60% more fat-free mass (e.g bones and muscles) compared to people who slept 8.5 hours per night .
7) Unhealthy Sleep Lowers Insulin Sensitivity and Increases Diabetes Risk
Glucose tolerance was significantly lower in sleep-deprived young men compared to those who slept well .
Sleep deprivation also results in reduced insulin sensitivity .
In a cohort study, men reporting a short sleep duration of less than or equal to 6 hours per night were twice as likely to develop diabetes as those men who reported 7 hours of sleep per night.
In the same study, men who reported a long sleep duration or greater than 8 hours of sleep per night) were three times as likely to develop diabetes than those who reported 7 hours of sleep per night .
Therefore, both short and long sleep durations increase the risk of developing diabetes .
Metabolic pathways linking Sleep disorders with the development of Type 2 diabetes
8) Poor Sleep Increases the Risks of Nonalcoholic Fatty liver Disease (NAFLD)
Nonalcoholic fatty liver disease (NAFLD) is a disease characterized by too much fat found in the liver of people who drink little to no alcohol which can lead to cirrhosis of the liver.
Short sleep duration and poor sleep quality were significantly associated with an increased risk of NAFLD .
While the Dongfeng-Tongji study showed that long nighttime sleep duration was also associated with a modestly increased risk of NAFLD 
Sleep and Cardiovascular Health
Sleep influences cardiovascular function in healthy people and in those with heart diseases . Getting quality sleep can reduce cardiovascular risks, while unhealthy sleep is associated with cardiovascular disorders .
9) Sleep Loss Increases the Risks of High Blood Pressure
A sleep duration of fewer than 5 hours per night was associated with a significantly increased risk of high blood pressure (hypertension) in subjects between the ages of 32-59 years old .
The increased risk for hypertension and increased sympathetic nervous system activity caused by lack of sleep may underlie the relationship between sleep deprivation and coronary heart disease .
10) Poor Sleep Increases Risks of Coronary Heart Disease
Both abnormally short and long self-reported sleep duration are independently associated with a modestly increased risk of coronary heart diseases .
Men who sleep 4 hours or less per night are more likely to die from coronary heart disease than those who sleep 7-7.9 hours per night .
Women reporting 9 or more hours of sleep per night had higher risks of developing coronary heart disease than those with 7 – 9 hours of sleep .
Sleep Balances the Immune System
Healthy sleep supports a healthy immune response, while a lack of sleep can worsen inflammation and autoimmunity.
11) Enhances Immune Responses
Sleep supports the Initiation of an adaptive Immune response
Good sleep also enhances the immune system’s ability to remember a previous infection and mount a more effective response in the case of re-infection [54, 56]. This is the methodology behind vaccines, and thus quality sleep improves the response to vaccination.
Sleep deprivation in humans changes the level of circulating immune cells (T cells and NK cells) and inflammatory cytokine levels (IL-1, IL-6, TNF-ɑ, etc.) . Increased levels of inflammatory cytokines can contribute to inflammatory diseases.
12) Sleep Loss May Trigger Autoimmune Disease
Sleep Deprivation Reduces Regulatory T Cell Activity
T regulatory cells (Treg) suppress inappropriate immune responses and prevent our immune system from attacking our own cells (self-tolerance) . The breakdown of the self-tolerance process can cause autoimmune diseases .
In experimentally sleep-deprived healthy people, Treg activity is reduced, suggesting that sleep deprivation may contribute to the development of autoimmune diseases .
Sleep Deprivation Increases Inflammation and Th17 Immune Responses
Sleep deprivation increases proinflammatory cytokines like IL-1, IL-1β, IL-6, TNF-ɑ, and IL-17.
Disordered sleep may induce systemic inflammation and activate Th17 cells, thereby leading to autoimmune diseases [60, 61, 62]. The Th17 cytokine IL-17 remained elevated following sleep deprivation, even after recovery for 7 days .
Read this post to learn more about Th17 and autoimmunity, and how to fix Th17 dominance.
Th17 activation has been associated with several autoimmune diseases including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), inflammatory bowel disease and multiple sclerosis .
Sleep is Important for Hormone Balance
13) Sleep Loss Activates HPA Axis and Increases Cortisol
The Hypothalamic-Pituitary-Adrenal (HPA) axis is the stress response axis which is activated in response to physiological or emotional stress .
Sleep, particularly slow wave sleep, inhibits the HPA axis and cortisol secretion .
Sleep loss and sleep disruption activate the HPA stress response axis, thus increases Corticotropin-releasing hormone (CRH) and cortisol levels [68, 69]. Read this post to learn about the negative effects of increased CRH.
14) Sleep and Thyroid Hormone Axis
The hypothalamus senses low circulating levels of thyroid hormone and responds by releasing thyrotropin-releasing hormone (TRH). TRH then stimulates the pituitary to produce thyroid-stimulating hormone (TSH). TSH further stimulates the thyroid gland to secrete the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine).
TSH levels increase with acute sleep deprivation but may decrease with chronic sleep deprivation . The increase in TSH suppresses reproductive functions.
GnRH= Gonadotropin-releasing hormone, LH= Luteinizing hormone, FSH=Follicle stimulating hormone
The Hypothalamic-Pituitary-Gonadal (HPG) axis mainly controls development, reproduction, and aging.
15) Sleep Loss Lowers Testosterone Levels
In young adult men, testosterone levels begin to rise upon falling asleep and reach peak levels at the time of first REM sleep. Once at its peak, testosterone levels remain at the same until awakening [77, 78]. The amount of increase in testosterone levels during sleep and decrease during time awake varied between individuals .
Sleep deprivation disrupts this natural testosterone level rise and fall. Total sleep deprivation reduces testosterone levels in men [80, 81]. This reduction may be dependent on age, with a greater reduction in older men [78, 82].
In a clinical cohort study, men with lower testosterone levels also had lower sleep efficiency .
16) Sleep Is Important for Healthy Estrogen Levels
Estradiol (a type of estrogen produced by the ovaries) is essential for the maintenance of healthy reproductive tissues in women.
In reproductive age women, partial sleep deprivation and a variable sleep schedule increased estradiol levels by approximately 60% compared to non-sleep deprived women .
However, in late reproductive age (perimenopausal) women, poor sleep quality is associated with lower estradiol levels .
In addition, both pre- and post-menopausal women with sleep-disordered breathing have lower estradiol levels compared to age- and cycle-matched women without sleep-disordered breathing .
17) Sleep Loss May Cause Infertility in Women
Sleep loss elevates several pituitary hormones, which interfere with normal reproductive functions in women.
In reproductive-aged women, an increase in TSH (as seen in hypothyroidism) due to sleep deprivation can cause several reproductive issues including problems with ovulation (anovulation), recurrent miscarriage, the absence of menstruation, and irregular menstrual cycles [87, 88].
In addition, high TSH levels can increase prolactin, which can also increase fertility. High prolactin is also associated with anovulation, polycystic ovary syndrome, and endometriosis .
High luteinizing hormone (LH, typically elevated in polycystic ovarian syndrome) can cause infertility. Healthy sleep keeps LH within normal, although fluctuating, levels . Sleep deprivation significantly increases the fluctuation of LH, with overall higher than healthy levels .
18) Sleep Increases Growth Hormone
In healthy young adults, medically induced slow-wave sleep resulted in an increase in growth hormone release .
Average growth hormone levels were higher during slow wave sleep compared to other sleep stages .
Growth hormone levels also increase during naps but increased more during afternoon naps than during morning naps .
Sleep Deprivation and Digestive Health
Sleep deprivation appears to worsen symptoms for all digestive disorders . During SWS smooth muscles in the colon contract less, so this phase of sleep is considered the “rest period” for the colon.
19) Stomach Ulcers
In animals, partial sleep deprivation has been shown to compromise stomach lining integrity by :
- increasing stomach acidity
- increasing blood levels of gastrin (a hormone that stimulates stomach acid secretion)
- increasing histamine
- increasing norepinephrine
- decreasing stomach mucosal blood flow
Sleep deprivation damages the stomach and could be one of the risk factors for ulcer formation.
Healthy sleep may protect against ulcer formation, as women who sleep more are less likely to get peptic ulcer disease .
20) Bowel Diseases
IBS patients symptoms decreased when subjects slept better .
In Crohn’s disease, the increase in IL-6 levels caused by sleep deprivation can worsen Crohn’s disease symptoms .
Further, a cross-sectional study found a nearly 3-fold increased risk of bowel disorders in patients with insomnia .
Sleep and Cancer Risks
21) Poor Sleep Increases Cancer Risks
Sleep problems such as difficulty falling asleep, problem maintaining sleep, poor sleep efficiency, early awakening, and excessive daytime sleepiness are prevalent in patients with cancer .
Meta-analysis studies suggest a positive association between long sleep duration and colorectal cancer .
Both extreme short and long sleep duration moderately increased the risk of colorectal cancer in postmenopausal women .
A cohort analysis in women suggests that longer sleep may be associated with increased risks of estrogen-mediated cancers .
Less than/equal to 6 hr of sleep increases the risk of developing prostate cancer, and sleep greater than/equal to 9 hr has a lower risk .
Sleep duration may influence breast cancer risk possibly via its effects on melatonin levels .
Melatonin prevents tumor initiation, promotion, and progression possibly by :
- Inhibiting abnormal/cancerous cell growth (anti-proliferative effects) [113, 114].
- Increasing p53, a tumor suppressor protein .
- Inducing cell differentiation 
- Preventing cancer cells from metastasizing (migrating and invading other tissues in the body) 
- Preventing angiogenesis (stopping cancer cells from growing their own blood vessels, which will allow the tumors to enlarge) 
- Decreasing telomerase activity, thus causing cancer cells to age and preventing the cells from dividing indefinitely [119, 120]
- Functioning as a free radical scavenger .
Melatonin production is closely related to sleep duration. Night-shift work disrupts sleep pattern and thus decreases melatonin levels, this may explain why the night-shift workers have higher cancer risks [122, 123, 124].
In addition, sleep also helps prevent cancer by:
- controlling inflammation [52, 53]
- maintaining healthy immune function 
- maintaining healthy levels of estrogen production (not too high) 
- maintaining healthy levels of oxidative stress 
Also, check out our new book Biohacking Insomnia ($).