“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:

Sleep is:

(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:

  • Insomnias
  • Hypersomnias
  • Sleep Related Breathing Disorders
  • Circadian Rhythm Sleep-Wake Disorders
  • Parasomnias
  • Sleep Movement Disorders

Each of these is sub-divided into further sub-categories. You may access the full classification via the link below.