“Sleep is the golden chain that ties health and our bodies together.”
– Thomas Dekker (c. 1572 – 1632)
Getting enough quality sleep is essential for good physical and mental health, and reduces the risk of ill-health. We are all well aware of the difference between a good night’s sleep and a poor one. Sleeping well makes us feel well. We are more energetic, more alert, and better able to concentrate and carry out our everyday activities.
When we don’t get sufficient quality sleep we are more likely to be moody and irritable, have problems with thinking, concentration, memory, and reaction times. This makes it harder to perform our daily activities and increases the risks of mistakes and accidents. However, chronic sleep loss can lead to problems far more serious than this.
Sleep isn’t just a period of time during which we’re not wake. Thanks to scientific research conducted over the past several decades it is now known that:
- Sleep is a very important, complex and dynamic biological process that’s essential for life.
- Sleep affects almost every type of tissue, organ, and system in the body.
- A chronic lack of sleep, or getting poor quality sleep, increases the risk of disorders such as hypertension (high blood pressure), cardiovascular disease, obesity, type-2 diabetes, and depression.
- During sleep, many important functions take place that do the following: help our body’s physical recovery and repair, support brain development, cardiac function and body metabolism, as well as support our learning, and improve our memory and mood.
Recent research findings also suggest that sleep plays a ‘housekeeping’ role in removing toxins from our brain that build up while we are awake. Researchers continue to explore the function and regulation of sleep. A key focus is to understand the risks involved with being chronically sleep deprived and the relationship between sleep and various diseases.
Lack of sleep is also known as sleep deprivation. It may be a consequence of short sleep duration or loss of sleep due to environmental factors. These could be, for example, noise or a contributing medical condition such as pain, diabetes, or mood/depression. Both too-short and too-long sleep durations are associated with higher risks of diseases and mortality. However, the complicated interactions that exist among lifestyle, mortality risk, and sleep duration are not currently understood.
What is sleep?
Most people could probably make an attempt at providing a somewhat vague and unscientific answer to this very difficult question. Even though, we should ideally spend about one third of lives sleeping, sleep remains quite a mysterious process. Good, relatively short, simple definitions of sleep are not easy to find in the scientific literature. However, one that I think simply and accurately describes ‘normal healthy sleep’ was proposed by Watson et al in the journal Sleep (June, 2015). This definition is as follows:
“Normal healthy sleep is characterized by sufficient duration, good quality, appropriate timing and regularity, and the absence of sleep disturbances and disorders.”
Essentially, good sleep health is characterized by a number of variables. These are: subjective satisfaction with sleep, appropriate timing and regularity of sleep, adequate sleep duration, high sleep efficiency, and sustained alertness during waking hours. Sleep efficiency is the ratio of the total time spent asleep in a night compared to the total amount of time spent in bed.
For hundreds of years, it was believed that sleep was characterized as being a period of brain inactivity. However, research conducted over the last 60 years has clearly demonstrated that the brain is actually very active during sleep.
Sleep scientists at Harvard Medical School have attempted to provide an answer to the question ‘What is sleep?’ by making a number of empirical points. These include the following:
(a) A state characterised by changes in physiological functions, including brain wave activity, breathing, heart rate, and body temperature.
(b) A period of reduced activity.
(c) Associated with a typical posture, such as lying down with eyes closed in humans.
(d) A state that is relatively easy to reverse. This distinguishes sleep from other states of reduced consciousness, e.g. hibernation and coma.
These scientists also note the following:
- Depending on the sleep stage, different physiological functions may be more active and variable.
- Sleep results in a decreased responsiveness to external stimuli.
Why do we sleep?
Scientists still aren’t entirely sure why we sleep. However, research over the past 20 years has started to provide at least a partial explanation. An important finding is that sleep doesn’t serve a single purpose. Instead, it appears to be needed for the optimal functioning of many biological processes. These include the inner workings of the immune system, proper hormonal balance, emotional and psychiatric health, learning and memory, and the removal of toxins from the brain. None of these functions fails completely as a result of insufficient sleep. However, as a general rule, sleep appears to enhance the performance of these systems instead of being absolutely necessary. Having said this, anyone who goes without sleep for an excessive period of time will die.
In attempting to answer the question ‘Why do we sleep?’, scientists have developed several promising theories. These theories include: Inactivity theory, Energy conservation theory, Restoration theory, and the Brain/neuro plasticity theory. Based on the evidence to date, it seems most likely that sleep will most likely be eventually explained on the basis of two or more theories. One of the most recent explanations for why we sleep is based on what’s known as neuroplasticity. According to this explanation, sleep is closely associated with changes in the structure and organization of the brain.
A sufficient amount of restorative sleep is vital for neuroplasticity i.e. the brain’s ability to adapt to input to occur. If our sleep is deficient, it becomes more difficult for us to process what we’ve learned during the day. We also have more trouble remembering it later. Researchers also believe that sleep may be involved in the removal of waste products from brain cells. This ‘housekeeping’ process appears to happen less efficiently when the brain is awake.
A little before we fall asleep, and during sleep itself, our central set temperature decreases slightly and we use less energy to maintain our body temperature. This being the case; it’s thought that conservation of energy is one of the functions of sleep. Another proposed function of sleep is that it allows the heart an opportunity to rest and recover from the constant demands placed on it during daily life. During non-REM sleep (non rapid eye movement sleep) there is an overall reduction in heart rate and blood pressure, as compared to when we’re awake. During REM sleep (rapid eye movement sleep), however, there are overall increases in heart rate and blood pressure.
What is Healthy Sleep?
In answering this fundamental question, the National Sleep Foundation (NSF) in the United States, provides 7 statements to describe the characteristics of healthy sleep. According to the NSF, if these apply to you, it’s a good sign that your sleep is “on track”. The 7 statements are:
- You fall asleep within 15-20 minutes of lying down to sleep.
- You regularly sleep a total of seven to nine hours in a 24-hour period.
- While in your bed, your sleep is continuous—you don’t have long periods of lying awake when you wish to be sleeping.
- You wake up feeling refreshed, as if you’ve “filled the tank.”
- You feel alert and are able to be fully productive throughout the waking hours (note, it’s natural for people to feel a dip in alertness during waking hours, but with healthy sleep, alertness returns).
- Your partner or family members do not notice any disturbing or out of the ordinary behaviour from you while you sleep, such as snoring, pauses in breathing, restlessness, or other sleep time behaviours.
In general, good sleep quality is associated with a sense of having slept continuously through the night and feeling refreshed and alert on wakening in the morning. Perception of sleep quality is subjective and varies considerably from one individual to another. Some individuals perceive their sleep as satisfying most of the time, while others consistently report being poor sleepers. However, sleep recording systems indicate that, in general, ‘poor sleepers’ tend to underestimate the length of time they sleep, as do some good sleepers. Having said this, there is some evidence to support a relationship between too-little or too-much sleep and the risk of cardiovascular diseases, such as atherosclerosis and myocardial infarction (heart attack), obesity, diabetes, depression, and even cancer. Whilst these risks are modest, they have been revealed in too many studies to reject the acknowledged effect of cumulative sleep debt on health maintenance.
Sleep Disorder Categories
The American Academy of Sleep Medicine provides a useful classification of Sleep Disorder Categories. It divides sleep disorders into the following six categories:
- Sleep Related Breathing Disorders
- Circadian Rhythm Sleep-Wake Disorders
- Sleep Movement Disorders
Each of these is sub-divided into further sub-categories. You may access the full classification via the link below.
Sleep & brainwaves
The instrument typically used to record brainwaves is an EEG (electroencephalogram) machine/device. It has electrodes that can detect brain activity when placed on a patient’s scalp. The electrodes record the brain wave patterns and the EEG machine/device amplifies the signals and sends the data to be recorded on a computer screen. The electrical activity appears on the screen as waveforms of varying amplitude and frequency. The German psychiatrist Hans Berger invented electroencephalography in 1924.
The four main brainwave states are Beta, Alpha, Theta, and Delta waves.
Beta brainwaves (12 to 38 Hz) dominate our normal waking state (consciousness) when our attention is directed towards the outside world. Beta activity is the fastest of the four main brainwave states. It’s the state that characterises when we’re engaged in mental activity such as decision making, focussing, making judgement, being alert and attentive. Beta waves were first described by the German psychiatrist Hans Berger. In 1929 he wrote “… that the beta waves not the alpha waves of the EEG are the concomitant phenomena of mental activity.”
Stages of sleep
Sleep is characterised by different stages that can be differentiated from each other by the patterns of brain wave activity that occur during each stage. These patterns are distinguished from one another by both the frequency and the amplitude of the brain waves.
What is called non-rapid eye movement (NREM) sleep is characterized by a reduction in physiological activity. As sleep deepens, a person’s brain waves slow down and gain amplitude, both breathing and the heart rate slow down, and the individual’s blood pressure drops. NREM sleep consists of 3 stages:
Stage N1 (formerly called Stage 1) is a transitional phase between wakefulness and sleep and it is characterised by both Alpha and Theta waves. It’s a time of drowsiness and people in this stage may occasionally experience sudden muscle jerks, preceded by a falling sensation. The early portion of N1 sleep produces Alpha waves, which are relatively low frequency (8–13Hz), high amplitude patterns of electrical activity. Alpha waves are dominant during quietly flowing thoughts and in some meditative states. As a person goes through N1 sleep, theta wave activity increases. Theta waves are even lower frequency (3 to 8 Hz), and higher amplitude than alpha waves. We are in N1 for a short time (one to seven minutes) before falling asleep. During this stage our eyes don’t move around a lot; they start to roll slowly and our muscle tone is still relatively high.
Stage N2 (formerly called Stage 2) is a period of light sleep during which eye movements stop. Brain waves become slower (theta wave activity) and there are occasional bursts of rapid waves (called sleep spindles). There are also spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate gets slower and body temperature decreases.
Stage N3 (formerly called stages 3 and 4) is often referred to as ‘slow wave sleep’ and is characterized by the presence of low frequency (0.5 to 3 Hz), high amplitude Delta waves. In N3, blood pressure falls, breathing slows, and temperature drops even lower, with the body becoming immobile. Sleep is deeper. There is no eye movement and decreased muscle activity is decreased.
Slow wave sleep is associated with bodily recovery, certain types of learning, and central nervous system changes. Normally, the amounts of slow wave sleep a person gets is directly related to accumulated sleep need. It’s thought that no one sleep state is more important than the other because each is necessary for life. Research in laboratory animals has shown that deprivation of any one of the stages is associated with decreased health and longevity.
Note: Hz (Hertz) is a unit of frequency i.e. the number of cycles per second.
REM stands for Rapid Eye Movement. REM is a distinct state, entirely different from the other ‘non-REM’ sleep stages. Our brainwaves are similar to those of wakefulness with fast beta wave activity. The existence of REM sleep was revealed in a technical paper co-authored by Eugene Aserinsky and his academic advisor Nathaniel Kleitman. The now classic paper was published in the journal Science in 1953. In this paper, the correlation of REM with dreaming was announced. Before REM was discovered it was assumed that sleep was a passive state and that the brain ‘switched off’ during sleep. This resulted in the opening up of an unknown and unexplored area of the sleeping brain to scientific research.
REM sleep is sometimes referred to as paradoxical sleep. It is characterised by the periodic recurrence of rapid eye movements, linked to a dramatic reduction in the amplitude of the EEG (electroencephalogram). The EEG of REM sleep closely resembles that of alert waking. Most dreams occur during REM sleep. It is the lightest stage of sleep and someone in REM sleep may wakeup easily.
Interestingly, a study published in the journal Neurology in 2017 found that individuals who developed dementia spent an average of 17 percent of their sleep time in REM sleep, compared to 20 percent for those who did not develop dementia. They also found that for every percent reduction in REM sleep there was a 9 percent increase in the risk of dementia. This means that the percentage of sleep time spent in REM sleep can be used as a predictor of dementia. During REM sleep, breathing becomes more rapid, irregular, and shallow. Heart rate increases and blood pressure rises. REM sleep is thought to play a role in memory consolidation, the synthesis and organization of cognition, and mood regulation. REM sleep begins about 90 minutes after falling asleep. The length of time spent in REM sleep increases as the night progresses, so that most of REM sleep occurs in the second half of the night.
The most mentally, and physically, restorative type of sleep occurs during stage N3. As mentioned earlier, sleep in this stage is called Delta sleep or slow-wave or deep sleep. It’s during this sleep stage that our bodies and minds go through the most renewal. Our heart rate and breathing rate slow to their lowest levels during sleep, and our muscles are relaxed.
Our body releases human growth hormone (HGH) which plays a vital role in cellular repair. Tissue repair and re-growth occurs. Also, the immune system is strengthened. About 15 to 25 percent of sleep time is spent in Delta/slow-wave/deep sleep which usually begins about 40 minutes after initially falling asleep. Typically, more time is spent in Delta/deep sleep during the first half of sleep. Of all of the sleep stages, it is the one least likely to be disturbed by external stimuli. Consequently, it is difficult to wake someone up when they’re in this stage of sleep. From a subjective point of view, deep sleep is considered to be the most refreshing part of the entire sleep cycle.
The circadian clock
The body’s natural clock is called a ‘circadian clock’. It’s also known as ‘circadian rhythm’. The primary circadian rhythm that this natural body clock controls is the sleep-wake cycle. It regulates the timing of such body rhythms as temperature, hormone levels, and other bodily functions. Abnormal circadian rhythms may be linked to obesity, diabetes, depression, bipolar disorder, seasonal affective disorder, and sleep disorders such as insomnia.
The circadian clock is “set” primarily by visual cues of light and darkness that are communicated along a pathway from the eyes to a part of the brain called the suprachiasmatic nucleus (SCN).
The SCN is located in the hypothalamus just above the optic chiasm. The SCN is responsible for controlling circadian rhythms. It keeps the clock synchronized to the 24-hour day. Other time cues, known as zeitgebers, can also influence the clock’s timing. A zeitgeber is an environmental agent or event (such as the occurrence of light or darkness) that provides the stimulus for setting or resetting a biological clock. Circadian rhythms and their sensitivity to time cues may change as an individual ages.
Sleep & aging
The need for sleep does not decrease with age. However, it is a common misconception that it does. Indeed, research demonstrates that our sleep needs remain constant throughout adulthood. In fact, older adults sometimes need more time for rest and recuperation after exertion. Having said this, older adults’ ability to sleep does decrease. They experience more fragmented sleep, in that they have an increase in the number of sleep stage shifts, arousals, and awakenings.
It’s thought that several factors may contribute to sleep problems later in life. These include:
- Decreased production of melatonin: According to the University of Maryland Medical Center, studies have indicated that melatonin production may decrease as you age. Melatonin is a hormone that helps regulate sleep. If less is melatonin is produced, sleep may be affected.
- Chronic medical problems: Medical conditions that may affect good quality sleep occur more frequently as we grow older. For example, although arthritis and back problems can develop at any age, they are more common in older adults. Both conditions can interfere with getting a good night’s sleep.
- Advanced sleep phase syndrome: This is a sleep disorder in which you fall asleep much earlier than a typical bedtime and wake up earlier. For example, if you used to go to bed at 10 p.m., but have developed a pattern of falling asleep at 6 pm and waking at 3 a.m., it might be due to advanced sleep phase syndrome. It occurs when the rhythm of your internal clock is off. It tends to occur more frequently in the elderly.
Sleep & women
Females are more prone to insomnia than men. In fact, according to a review titled ‘Sex Differences in Insomnia: from Epidemiology and Etiology to Intervention’ published in the August 2018 issue of Current Psychiatry Reports, insomnia is approximately 1.5 times more common in women than in men. This increased proneness of 1.5 times more is very much in line with that cited in a meta-analysis titled ‘Sex differences in insomnia’ published in the journal Sleep in 2006. According to this meta-analysis, women are 1.41 times more predisposed to suffer from insomnia than men at all ages, but especially at advanced ages. Also, according to this meta-analysis, menopausal women had a poorer sleep quality than non-menopausal women.
Sleep is particularly disturbed in women during pregnancy and menopause. In addition, in general, women are more likely to be involved than men, in activities such as the night-time care of babies, young children, and sometimes elderly relatives. This can significantly contribute to lack of sleep and poor quality sleep.
In addition, many women, especially younger ones, are pulled in different directions by the demands of work, family, and home life. As a consequence, they are prone to night-time worries and mental activity that can prevent them from getting to sleep.
Sleep and men
In men, there is a connection between testosterone levels and sleep. When men fall asleep their testosterone levels increase. Levels continue to rise for about the first 90 minutes at which point they plateau somewhat. They remain high and eventually peak in the morning around about 8 am. They then start to decrease during the course of the day, and reach a low around 8 pm.
Erections occur in healthy males when they’re asleep, mainly during REM (rapid eye movement) sleep. This continues through to old age. These erections usually last for about 20 minutes but their duration and firmness decrease as the man ages.
Although erections occur during REM sleep, the stage during which most dreaming occurs, they are unrelated to the content of the dreams. The function of sleep-related erections isn’t known. However, it has been speculated that they may serve to provide life-long exercise for the penis.
Sleep and temperature
Most people sleep better in a room that’s around 65 degrees Fahrenheit or 18 degrees Celsius. The National Sleep Foundation in the United States recommends this temperature and says that sleep is disrupted if the temperature rises above 75 degrees Fahrenheit or drops below 54 degrees Fahrenheit.
The human body has a temperature cycle in that our core body temperature falls during the night, bottoming out a few hours before we awake and then increasing as we approach morning. A cool bedroom is a key factor in getting a good night’s sleep.
Sleep & alcohol
Alcohol is effective in inducing sleep but it impairs sleep during the second part of the night. It is a powerful suppressor of REM sleep. Alcohol is a sedative. However, sedation is not sleep and it doesn’t induce natural sleep. It fragments sleep with numerous brief awakenings and as a result it’s not restorative. Another effect of alcohol is that it interrupts sleep due to the fact that it is a diuretic and tends to result in an increased number of sleep disrupting toilet visits. It’s also known that alcohol can increase the severity of obstructive sleep apnoea (OSA). It can also cause snoring and induce apnoeas in people without a history of OSA.
Effect on alcoholics
An interesting study reported in the journal Sleep in 2009 looked at the effect of alcohol on the sleep of alcoholics as compared to non-alcoholic control subjects. The findings showed that in long-term alcoholics who hadn’t had a drink for up to 719 days, the percentage of slow wave sleep was significantly lower than in controls. It was also found that alcoholics also had significantly more stage N1 non-rapid eye movement (NREM) sleep than controls. According to the researchers:
“……….. having less deep, slow wave sleep and more light, stage 1 sleep is reflective of poorer sleep quality, which could act as an exacerbating factor in alcoholics’ cognitive decline.”
Sleep & nicotine
Nicotine disrupts sleep mainly because it is a central nervous system (CNS) stimulant. It also causes a release of the hormone epinephrine (adrenaline), which further stimulates the nervous system. Although nicotine may make some people feel drowsy or less anxious and more relaxed, the vast majority respond to its CNS stimulant effects.
Another reason why nicotine can disrupt sleep is because smoking (consuming nicotine) is a risk factor for obstructive sleep apnoea (OSA). Indeed, smokers are three times more likely to have OSA than are people who’ve never smoked. Also, interesting is the following:
A large, longitudinal study published in 2019 in the journal Sleep investigated the effect on sleep of nicotine, caffeine or alcohol consumed within 4 hours of bedtime. The researchers found that:
- Nicotine was the substance most strongly associated with sleep disruption.
- There was a statistically significant interaction between evening nicotine use and insomnia in relation to sleep duration.
- Among participants with insomnia, nightly nicotine use was associated with an average 42 to 47-minute reduction in sleep duration.
The results are particularly relevant because they were observed in individuals who prior to the study, generally had high sleep efficiency.
A study carried out in 2007 by the Monell Chemical Senses Center, reported that nicotine in the breast milk of lactating mothers who smoke cigarettes disrupts their infants’ sleep patterns.