The Effects of Sleep on Health

Until recently, sleep was perhaps the under-recognised piece of the health puzzle. It’s not that it was completely ignored, but the conversation generally revolved mostly around diet and exercise. Now, we are beginning to see, more and more, the enormous role that healthy sleeping patterns play in our overall health and our performance. Consequently, there is a large and growing body of research that investigates the roles that sleep plays as both a contributor to health and disease and the ways that it helps to improve our health and performance.

What happens when we sleep?

When we sleep, our body reduces our sensory and muscle activity and our consciousness goes into an altered state. During sleep, we go through several phases of sleep. These are broadly categorised as rapid eye movement (REM) and non-REM sleep.

During sleep, due to the reduced overall activity, the body can accelerate its recovery by building tissue and restoring immune and hormone functions. It has also recently been discovered that sleep is critical to remove toxins created by normal day-to-day functioning from the brain. These metabolites are not removed during the day but when we sleep are rhythmically ‘washed’ from the brain and central nervous system by the glymphatic (glial + lymphatic) system. Much of the body’s rebuilding, repair, and waste removal is carried out during REMsleep.

Sleep is critical to remove toxins created by normal day-to-day functioning, from the brain

The cycles of sleep

Within minutes we enter stage one sleep, characterised by alpha and theta brain waves. After several minutes of light, stage one sleep, we enter stage two. This stage is characterised by ‘sleep spindles’, bursts of neural activity.

Deep sleep (also called slow-wave sleep) involves stage three and four. In these stages, muscle activity is inhibited, and it is harder to awaken. The brain produces more delta waves. In this stage is when much of the repair of the body takes place and so, getting enough deep sleep is critical to health.

About 90 minutes after falling asleep we enter REM sleep characterised by rapid, jerking movements of the eyes. In this phase, the brain reactivates (most dreaming occurs during REM sleep). REM sleep plays an important role in learning and memory function, as it allows the brain to process and organise learning and memory.

How much sleep should we get?

According to the National Sleep Foundation of the US, which convened an expert panel to evaluate optimal sleep times, the recommended amounts of sleep for various ages are¹:

• 0-3 months: 14-17 hours per night
• 4-11 months: 12-15 hours
• 1-2 years: 11-14 hours
• 3-5 years: 10-13 hours
• 6-13 years: 9-11 hours
• 14-17 years: 8-10 hours
• 18-64 years: 7-9 hours
• 65+ years: 7-8 hours

However, the quality of sleep is also extremely important, with adequate periods of deep sleep and REM sleep. While there is no consensus on exactly how much REM and deep sleep we require to function optimally, deep sleep should account for at least 13% of total sleep duration, while REM sleep typically accounts for at least 20% of sleep in healthy people.²

There can also be significant variations between people, and while most of us probably do best around the norms suggested above, it is known that variability between people exists and some thrive on greater or lesser sleep times and different sleep patterns.

Is sleeping in our genes?

Twin studies have shown that a substantial proportion of the variance for sleep variables is due to genetic factors with around 30-40% of the variability in sleep quality and duration attributed to ‘nature’ vs nurture.³ So, that means that although many of our sleep traits are influenced by our genes, far more is influenced by our behaviours and lifestyle.

The effects of sleep deprivation on health

Both short and long sleep durations are associated with increased mortality risk in adults and the elderly.^4-6

Short sleep is associated with increased mortality risk.[i] It is also associated with an increased risk of ⁴:

• Diabetes mellitus (RR, 1.37; CI 1.22–1.53)
• hypertension (1.17; 1.09–1.26)
• cardiovascular diseases (1.16; 1.10–1.23)
• coronary heart diseases (1.26; 1.15–1.38)
• and obesity (1.38; 1.25–1.53)

Long sleep durations are also associated with mortality,[ii] diabetes, cardiovascular disease, stroke, coronary heart disease, and obesity.⁶

It is important to note that correlation does not equal causation. Illnesses and health conditions can alter sleep patterns (through physical pain and discomfort or mental and emotional anguish) but conversely, poor sleeping patterns can also alter physiological processes that precipitate poor health. There is likely bidirectionality in which sleep can worsen health, and health conditions can also worsen sleep, leading to a vicious cycle of both poorer health and poorer sleep.

Sleep can worsen health, and health conditions can also worsen sleep

The Effects of Sleep on Health Conditions

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Arthritis and back pain

Sleep interventions that improve sleep, including cognitive behavioural therapy (and medication), reduced insomnia and pain in people with low-back pain.⁷

Attention-deficit/hyperactivity disorder

While findings are mixed, sleep disturbances are associated with worsened clinical and cognitive outcomes in adolescents with ADHD.⁸

Cardiovascular disease

Systematic reviews have identified the lowest risk level for cardiovascular disease of ~7 hours of sleep per night. Sleep durations less than 7 hours are associated with a 6% increase in risk per hour of sleep lost. Sleep durations over 7 hours per night are associated with a greater risk of approximately 13% per 1‐hour increase.⁹ Other reviews have found similar results with both long and short durations of sleep associated with significantly increased risk of stroke.¹⁰

Cognition and brain health

Lack of sleep is known to worsen cognitive abilities and particularly, sleep improves the ability to store memories for later retrieval.¹¹

In children

A significant effect has been found between sleep duration and cognition. Long durations of sleep in children are associated with better cognitive functioning. It has been shown that IQ is significantly associated with sleep loss, but memory, processing speed and attention are not. Worryingly, in this review of the evidence, it was shown that in most children studied, sleep durations were not within the range of recommended sleep. As such, the effects of loss of sleep on cognition may be underestimated, as most children were already sleep-restricted.¹²

In adolescents

Adolescents are considered to be at risk for deteriorated cognitive functioning due to insufficient sleep. It is known that inadequate sleep results in changes to the structure and function of the brains of children and adolescents.¹³ Additionally, both objective and subjective reductions in sleep quality increase stress and cortisol levels which impact sleep duration and quality.¹⁴ In a review of the effects of experimental sleep manipulation on adolescents, partial sleep restriction was shown to have little to no effect on adolescent cognitive functioning but total sleep deprivation was found to reduce alertness and task completion, while both sleep extension and sleep improvement improved working memory. It has also been shown that sleeping directly after learning improves the ability to retain that learning.¹⁵

In older adults

In meta-analyses of sleep duration in older adults:

• Self-reported short and long sleep was associated with poorer cognition
• This was shown in both cross-sectional and prospective studies
• Associations of short and long sleep were found across multiple measures of cognition

Both short and long sleep duration are associated with a greater than 40% increase in poorer cognition scores.[iii] In cross-sectional studies of sleep duration and cognition, extremes of sleep duration were significantly associated with poorer performance over multiple domains (such as performance, executive function, verbal memory, and working memory).¹⁶ Those with sleep-disordered breathing were 26% more likely to develop cognitive impairment.¹⁷

Dementia

People with dementia tend to have disturbed sleep and while this could be related to dementia causing the sleep disturbance, it is likely to be bidirectional and studies suggest that sleep disturbances can increase the risk of dementia. Compared with people without sleep disturbances, those with disturbed sleep have a higher risk of dementia and Alzheimer’s disease. For example, people with sleep problems have been demonstrated to have a 68% greater likelihood of cognitive impairment or Alzheimer’s disease[iv] and approximately 15% of Alzheimer’s disease in the population may be attributed to sleep problems.¹⁸

• Subgroup analysis has also shown that insomnia increases the risk of Alzheimer’s disease but not vascular or all-cause dementia
• Sleep-disordered breathing was associated with a higher incidence of all-cause dementia, AD, and vascular dementia¹⁹

In a meta-analysis of studies including over 69000 participants, individuals with sleep problems had around 55% to 400% greater risk of a range of cognitive challenges including Alzheimer’s disease, cognitive impairment, and preclinical Alzheimer’s disease than people without sleep problems.¹⁸

Diabetes

Sleep disturbances including short (less than 6 hours) and long (greater than 8 hours) sleeping time, insomnia (initiating or maintaining sleep), sleep apnoea, and abnormal sleep timing have been associated with increased diabetes risk,²⁰ and both long and short sleep are associated with increased HbA1c (a measure of average blood sugar levels).²¹ Risk of diabetes is associated with increases of 48% for people sleeping less than 5 hours a night and 36% for those sleeping more than 9 hours. Sleep apnoea has shown the strongest association with diabetes, more than doubling risk, while poor sleep quality and shift work both increase risk by ~40%.²⁰

Inflammation

Over 70 studies featuring more than 50000 participants have evaluated the effects of sleep deprivation on inflammation. Sleep disturbance was associated with higher levels of c-reactive protein, and the inflammatory marker interleukin 6 (IL-6). Shorter sleep duration was associated with higher levels of c-reactive protein, but not IL-6. Long sleep durations (>9 hours) were also associated with increased inflammation marked by higher c-reactive protein, and IL-6. Interestingly, neither long nor short sleep or sleep disturbance were associated with increased levels of tumour necrosis factor-α, one of the key markers of autoimmune inflammatory conditions like Crohn’s disease.²²

Note: The addition of a multi-nutrient in a higher-protein diet (1.5 g/kg/bw per day) and containing arginine, glutamine, zinc sulphate, vitamin C, vitamin D3, and omega-3 fatty acids compared with lower protein intake (0.8 g/kg/bw) ) did not significantly mitigate the reduced wound-healing resulting from shortened sleep but did improve immune/inflammatory responses after skin injury (as shown by higher post-injury IL-6 and IL-8 concentrations).²³

Mental Health

Sleep quality, duration of sleep, and sleep latency (the time take to get to sleep) are known to affect daytime mood.²⁴ Sleep disturbances also result in significant increases in anxiety.²⁵

• Sleep deprivation results in significantly increased in anxiety levels, but sleep restriction does not
• Conversely, while not significant, there is also a trend towards greater anxiety with extremes of long sleep²⁶

Preliminary evidence also suggests a link between sleep disturbances and psychosis. This association may be partly causative and resultant (from neurocognitive anomalies and symptom severity).²⁷

Among older adults, there is a high prevalence of sleep disturbance and depression. Evidence suggests a bidirectional relationship between these, with poor sleep contributing to depression and depression encouraging poorer sleep.²⁸

Bipolar disorder is also associated with sleep disturbances.²⁹ Individuals at high risk of bipolar disorder report greater incidences of the irregularity of sleep/wake times, poor sleep, and circadian rhythm disruption. It has been suggested that poor sleep quality, night-time awakenings, and inadequate sleep are possible predictive factors for bipolar disorder. ³⁰

Risk-taking and suicide

Sleep disturbances, insomnia, and nightmares all show an association with suicide risk.^31-33 This might be affected by an elevation in impulsivity which could increase the tendency to act of suicidal ideation.³³ A systematic review (26 studies, 579,380 participants) indicated that insufficient sleep was associated with 43% greater odds of risk-taking across diverse categories of risk, including alcohol and drug use, smoking, violent or delinquent behaviour, transport risk-taking/road safety, and sexual risk-taking.³⁴ Sleep duration may also affect the risk of suicide. The lowest association with suicidality is seen between 8 and 9 hours of sleep per night, with every 1-hour increase in sleep associated with an 11% decrease in suicidal ideation.³²

Multiple sclerosis

It is an almost universal finding that sleep disturbances have significant associations with cognitive dysfunction in multiple sclerosis. In studies, self-reported sleep disturbance generally predicted self-reported (but not objective) measures of cognitive dysfunction. Conversely, objective sleep measures (e.g., polysomnography, actigraphy) generally predicted objective impairments in processing speed and attention; however, objective sleep disturbance was more variable in predicting performance in other cognitive domains (e.g., memory, executive function).³⁵

Obesity

Sleep duration is likely to influence weight gain and overweight/obesity and resulting health effects.³⁶ Meta-analyses have revealed that reduced sleep duration is associated with between 30-71% greater risk of obesity in children. ^36-38

Sub-group analysis shows that short sleep duration is associated with a greater risk of developing overweight or obesity in:

• infancy (seven studies, 14738 participants, risk ratio [RR] 1.40; 95% CI 1.19 to 1.65; p < .001)
• early childhood (eight studies, 31104 participants, RR 1.57; 1.40 to 1.76; p < .001)
• middle childhood (three studies, 3005 participants, RR 2.23; 2.18 to 2.27; p < .001)
• and adolescence (three studies, 26652 participants, RR 1.30; 1.11 to 1.53; p < .002).³⁶

While the reasons for this associated are not entirely clear, in children a link has been found between short sleep duration and the development of insulin resistance, sedentary behaviours and unhealthy eating patterns.³⁹ It has also been shown that partial sleep deprivation results in increased energy intake (i.e. you eat more) and that people overeating as a result of sleep deprivation tend to eat more fat and less protein,⁴⁰ and have a poorer diet overall,⁴¹ characterised by more snacking and soda use.⁴² This relationship is also bidirectional, with a poorer diet likely to lead to poorer sleep.⁴¹ Conversely, getting adequate sleep is associated with a higher intake of fruits and vegetables.⁴²

Neurodegeneration

An extremely strong connection between sleep disorders and neurodegenerative diseases has been drawn. For example, in those with a REM sleep disorder (disabling the ability to achieve proper REM sleep), the risk of developing a neurodegenerative disease was 33.5% at five years follow-up, 82.4% at 10.5 years and 96.6% at 14 years, with nearly half of these developing Parkinson’s disease.⁴³

Conclusion

Sleep has a ‘bi-directional’ relationship with health. That is to say, sleep affects health, poor health affects sleep, and sleep affects and is affected by the foundations of health. For example, when we eat poorly, don’t move, are stressed, or less mindful, our sleep suffers, conversely, when we sleep poorly, we don’t eat or move as well, and we are less mindful and more prone to stress.

On balance, ensuring that we get enough high-quality sleep is critical to health, performance, and happiness.

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[i] Overall risk ratio (RR) 1.12; 95% confidence interval (CI) 1.08–1.16

[ii] RR 1.39; 95% CI 1.31–1.47

[iii] OR 1.40; 95% CI 1.27–1.56

[iv] RR 1.68; 95% CI 1.51–1.87