Poor Sleep as a Risk Factor of Developing Dementia

Why is sleep so important? 

Sleep and waking state rhythms are essential parts of life. We sleep to recover. But sleep is also a lot more than just a restful respite from work or other important activities. It is the unobtrusive process that remaps, reshapes, and restores our brain in more ways than one to help us to function efficiently throughout the day. And there is a reason why we spend one-third of our life asleep: sleep supports healthy ageing.

Sleep is important for short-term memory performance and memory consolidation. Good sleep stimulates an entire range of cognitive functions such as attention and reasoning, influencing our emotional balance and the ability we have to control ourselves – all of which are important conditions for health. On the other hand, poor sleep can impair intellectual and cognitive performance, which can cause a lack of concentration, poor learning and impaired memory, and can trigger cognitive decline.

Indeed, negative sleeping patterns can affect our physical and mental abilities because sleep is essential to brain maintenance and homeostatic balance. Poor sleep and sleep deprivation, either caused by societal or behavioural factors, are the reason why sleep loss can lead to not only dementia, but other mental and physical illnesses such as depression, heart disease, or obesity. Let's review some of the reasons why sleep is so important for our brain health and why and how it is associated with the onset of dementia.

What should you know about sleep?

Circadian rhythms

Before we start, we need to understand how sleep works. Sleep occurs during a person's normal 24-hour sleep cycle which is guided by circadian rhythms that propel us to sleep and wake through biological and genetic drivers (molecular clock) also called circadian drive. Circadian rhythms are physical, mental and behavioural changes that follow a roughly 24-hour cycle, responding primarily to the light and darkness of the environment. These rhythms are triggered by genetic mechanisms in our brain which synchronise and feedback with the biorhythm mechanisms within every organ. 

Biological mediators of sleep homeostasis

Melatonin and cyclic adenosine monophosphate (cAMP) are key mediators of sleep homeostasis. In humans, melatonin is a hormone produced in the pineal gland acts as an essential driver of this rhythm, responding to sunlight and darkness cues. cAMP is a messenger of biological function. Melatonin production is increased at night (<18 h) and slowly increases our relaxation levels and our propensity to enter a new sleep cycle, while adenosine increases sleep pressure by binding to brain receptors and increasing the feeling of drowsiness (a condition affected by caffeine intake for example).

Three stages of sleep cycle

Sleep is in itself made of different cycles that are usually divided into three different chronological stages:

  • light sleep
  • deep sleep
  • dreaming

These are called the different stages of sleep. They form a physiologically imposed routine that repeats itself every night. Each sleep stage has important functions for maintaining brain function and homeostatic balance.

The length of sleep of each stage, as well as the total length of sleep throughout the night, varies from person to person. And although the normal total amount of sleep is usually considered to be around 8 hours for normal adults (between 12 and 15 hours for children and 10 hours or more for adolescents), it is important to consider our own predisposition to sleep, or the amount of sleep we need, as some people may need more sleep or fewer hours of sleep. 

How much sleep does one need?

The amount of time one needs to sleep depends on what is called their chronotype – a person's natural inclination with regard to the times of day when they prefer to sleep or when they are most alert or energetic – as well as their natural sleep duration. Such predisposition is triggered by genetic mechanisms that seldom change in adult life, although such rhythms start to be modified after approximately 50 years of age. As such, people who naturally sleep less may have a genetic predisposition to do so. But that condition is rare. Only less than 1% of the population can sleep around 5 hours per night without suffering any negative brain health consequences. 

Most of us need around 8 hours. But to the same extent that sleep deprivation leads to brain health issues, sleeping too many hours, or sleeping consistently longer hours than we need for our chronotype is also detrimental to the brain's health. 

Knowing our chronotype is essential to our overall health. Research shows a reduction of the hippocampus – an area of the brain related to short-term memory – after chronic “jet lags” (or after regularly changing circadian rhythms). Similarly, studies of people who did not respect their sleep pattern or natural body clock indicate they are more likely to be affected by illnesses such as depression but also reported lower levels of psycho-emotional wellbeing.1 And, in the worst spectrum of that condition, sleep deprivation is currently seen to onset neurodegenerative diseases. 

These considerations need to be weighed very seriously by any individual to allow for good sleep quality to take place. Good sleep is a necessity for our survival: one that is of good quality and with the correct duration are necessary for good brain health.

What are the benefits of sleep?

When we sleep, our brain goes through rhythmic patterns associated with specific brain and physiological functions that allow for improved cognition and biological performance. Sleep helps to:

  • Increase immunity of sleep function
  • Lower risk of infection
  • Enhance memory consolidation
  • Increase brain health by clearing the brain of toxic proteins
  • Replenish energy
  • Promote recovery from stress 
  • Increase neural plasticity (the brain’s ability to rewire itself)

It is specifically within that general physiological context that poor sleep, such as sleep deprivation, causes gradual cognitive impairment and later results in neurodegeneration as a result of physiological disruptions to these mechanisms. For example, studies of REM disruptions have shown how such conditions lead to effects on anxiety, irritability, and difficulty in concentrating after dreams are consistently interrupted. And drugs such as Selective Serotonin Reuptake Inhibitors (SSRIs), although important for depression, suppress REM and, therefore, when consistently used could increase such effects in the long-term. 

But many other, more complex mechanisms are at play while we sleep: these range from the regulation of the circadian rhythms by signalling pathways in the hindbrain, hypothalamus, and pituitary that are triggered by retina exposition to light, to biological factors associated with cortisol levels, growth hormones, insulin levels and other neurophysiological factors. As such, poor sleep quality is equally known to negatively affect our blood pressure, glucose and insulin levels. All of which may be linked to risk factors of dementia.

What are the consequences of poor sleep?

Poor sleep is any sleep pattern that is behaviourally negative to sleep: either leading to sleep deprivation, too much sleep (e.g., too many hours sleeping), sleep disruption or sleep disturbance. It may be imposed by societal needs when they are incompatible with our chronotype (e.g., having to wake up very early when we have a chronotype of a night owl), or behaviourally maintained (but are incompatible with sleep).

Many studies have demonstrated the effect of poor sleep. These effects are associated with short-term, long-term and even psychiatric effects, all of which can lead to potential long-term brain health consequences. 

Short-term effects of poor sleep:

  • Loss of attention 
  • High level of microsleeps (increase day-dreaming or events of micro-sleep during the day)
  • Failure to process information (e.g., reduction of the capacity of memorisation).
  • Impulsivity (e.g., increase lack of control, and short-temper)
  • Memory impairment
  • Reduced cognition and creativity
  • Reduced athletic performance

Long-term effects of poor sleep

  • Immune suppression
  • Increased risk of infection or cancer 
  • Increased risk of cardiovascular disease
  • Increased risk of ischemic stroke (due to clot formation)
  • Risk of developing diabetes II
  • Risk of developing metabolic syndrome and obesity
  • Risk of developing hypothyroidism
  • Risk of developing dementia (dementia risk) or alzheimer’s disease

Psychiatric effects

  • Increased risk of mood instability
  • Increased risk of developing anxiety
  • Increased risk of stimulant or sedative use
  • Increased risk of having a mental health crisis, such as falling into depression.

It is important to note that poor sleep is also associated with oversleeping. Sleeping too much — e.g., 10 hours or more, unless it is your chronotype — is known to be equally harmful to your health. And although it may be equally a sign of an underlying health condition, a multiethnic cohort study indicated that both, shorter (≤5h/day) and longer (≥9h/day) sleeper, for healthy individuals with no history of cancer or heart disease, had an increased risk of cardiovascular disease mortality for both sexes.2 And sleeping too much has been theorised as being of a greater risk to health for coronary heart disease, stroke and diabetes than sleeping too little.

As such it is important to understand the characteristics and signs of poor sleep, and what is considered to be poor sleep.

What is considered poor sleep quality?

How can we spot the signs of poor sleep quality? Any disruption in the normal sleep structure of the person, when the person does not follow the normal drivers of his or her biological clock, can be considered to impact the sleep quality and efficiency of the person's sleep. But what are the disrupters of good sleep? And what are the habits that can lead to bad sleep quality?

A person's sleep-wake cycle can be disrupted by a number of physical and environmental changes, as well as behavioural incompatible sleep conditions, including:

  • regularly changing circadian rhythms (e.g., jet-lags)
  • noise pollution
  • playing during the night or television (due to blue light exposure before sleep)
  • Increase coffee or theine intake
  • Unmoderated alcohol intake

Coffee prevents adenosine (mentioned above) from functioning and reduces sleep pressure. And the mean half-life (the time required for the concentration of a substance in the body to decrease by half) of caffeine in the plasma of healthy individuals is about 5 hours, and such, caffeine can take 9.5 hours to be eliminated from the blood (where the caffeine effect is no longer seen). More importantly, caffeine is known to reduce the depth of deep sleep affecting its quality. Alcohol intake is also associated with disrupting the sleeping stages (sleep architecture of the different stages of sleep). For example, contrary to what is believed, alcohol intake prior to sleep reduces the time it takes to fall asleep (sleep onset latency) and disrupts your sleep patterns (sleep architecture) as well as leading to a more disrupted sleep throughout the night and as such leads to poor sleep quality (sleep efficiency). 

What are signs of poor sleep (not respecting the rhythms of our molecular clock)?

  • short sleep duration or sleep duration below our normal need (based on your chronotype)
  • sleep too much (based on your chronotype)
  • feeling drained after having more than adequate sleep (which can be a sign of obstructive sleep apnea or thyroid problem)
  • feelings of brain fog throughout the day (not being able to concentrate as usual; which is different from daytime sleepiness)
  • daytime sleepiness (feeling sleepy)
  • reduction of productivity or underperforming (often a consequence of sleep deprivation)

Although, poor quality sleep is usually a result of a combination of factors, rather than just one, any of such signs need to be taken seriously. And although some of them may be due to a lack of knowledge associated with the causes that lead to such conditions, these need to be acted upon, based on your chronotype, to avoid any of the long-term effects of bad sleep habits.

What is considered good sleep?

Good sleepers naturally follow the rhythms of their natural clock, establishing a routine that gives echo to these rhythms. They listen to their body's natural predisposition as much as they minimise the behavioural effects that impact the nature of these rhythms.  As indicated by Colin A. Espie, a clinical director of the Nuffield Department of clinical neurosciences in Oxford: "The good sleeper is not thought of as following a recipe to produce the perfect sleep. Rather, the good sleeper is regarded as essentially passive because internal and external cues act as automated setting conditions for sleep... The good sleeper sleeps just as he walks, or talks—without thinking about it".3

Having said that, good sleepers have specific behavioural characteristics that do help them to be good sleepers:

  • Good sleepers have a regular amount of sleep hours throughout the week. This includes a regular amount of sleep without changes of patterns (e.g., time of sleep) over the weekend (when not working) or during vacations.
  • Good sleepers sleep at regular hours and follow their biological clock propensities.
  • Good sleepers avoid food intake before sleep (to avoid increasing the metabolism before sleep and disrupting the slow metabolic activity of deep sleep).
  • Good sleepers know the effects that stimulants such as caffeine have in their sleeping patterns and stop caffeine intake during days where it is not needed.
  • Good sleepers avoid regular alcohol intake.

The detrimental associations of poor sleep to different and cause-specific health risks are clear, from risk to cardiovascular disease, cancer mortality risks and the development of dementia. Consequently, as indicated by The British Psychological Society: "Sleep optimization should be considered a priority across the health spectrum… with the potential to improve the management of long-term conditions and, more broadly, societal health and well-being". Being forewarned is being forearmed.


  1.  Why Fighting Your Natural Body Clock May Increase Your Depression Risk. Healthline [Internet]. 2021 [cited 2023 Jul 25].
  2. 11 Effects of Sleep Deprivation on Your Body. Healthline [Internet]. 2020 [cited 2023 Jul 25].
  3. Why Irregular Sleep Patterns May Affect Your Heart Health. Healthline [Internet]. 2020 [cited 2023 Jul 25].
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Rodrigo Montenegro

Master of Science - MSc. Neuroscience, Universidad Isabel I, Spain

Rodrigo Montenegro is a Neuroscientist with Sleep Medicine specialization from Oxford University. Rodrigo has worked as a lead Neuroscientist developing a clinical grade sleep-headband and as a consultant in applied medical neuromodulation technologies.

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