Sleep encyclopedia

A good night's sleep is important, not just because roughly one third of our lives is spent in sleep, but also because this third of our life determines the quality of the other two thirds.

This is why it is important to make use of available resources to ensure the best possible sleep experience. This sleep encyclopedia is intended as a reference book for those who would like to know more about sleep and who by means of improved sleep would like to achieve improved quality of life. 

The encyclopedia is absed on psychiatrist Jes Gerlach's book 'Sleep' (2003). PsykiatriFondens Forlag.


Apnoea is snoring caused by a lack of oxygen, which can result in several hundred minor awakenings in the course of the night. These are not always registered by the sufferer. Sleep thus becomes extremely superficial, resulting in drowsiness, irritability, headaches and lapses of memory.  
The condition can be life threatening, as there may be a risk of insufficient oxygen due to the long breathing intervals. As mentioned earlier, the sufferer may not register their irregular breathing. If you suspect apnoea, however, you should visit a doctor. 
Apnoea is treatable and changes in lifestyle such as weight loss, avoidance of alcohol and sleeping medicine or learning to sleep on your side or the use of a tooth guard or even surgery are all possible options.




Dreams are a kind of nocturnal fantasy that everyone experiences every few hours during the night, even if you are unable to remember them the next morning. To a certain extent, dreams reflect our experiences; human contact, feelings, values and conflicts. 
The most vivid dreams occur during REM sleep, which is characterised by more emotional dreams than those of other sleep phases. People often awake in the morning after an REM phase and can, if they try, remember fragments or the tailend of a dream. Between 10 and 20% remember nothing of their dreams, but it may help to lie still and try and stay in the dream and in this way re-experience it. 
The question still remains, however, as to what importance dreams have and why they occur. Previously, psychotherapists and others believed that dreams were the path to the unconscious and thus an important tool for gaining insight into the problems of the mentally ill. Today, debate has taken a different route, posing the question whether dreams play an important role in learning and memory. Research, however, has not yet provided an unequivocal answer.



History of sleep

In Ancient Greece, the God of sleep was known as Hypnos. He and his brother Thanatos, the God of Death, were both sons of the God of Night, Nyx. As far back as Ancient Greece, man has realised the importance of sleep for his survival.

In his time, the philosopher Aristotle invented a scientific theory which claimed that the organism needed to turn food into inner gasses, which in turn produced sleep.




At some point in life, most people experience sleeping problems. However, 10-15% of the population suffer from severe, chronic insomnia.
Particularly people suffering from psychological illnesses, including depression, experience insomnia either as a symptom of the illness or as a direct result of the symptoms themselves. 
Many elderly people also suffer from insomnia, which is primarily due to a change in sleep patterns, just as physical and psychological illnesses as well as a higher intake of medicine can lead to insomnia. 
If you or your immediate family experience sleeping problems, the first step is to use your common sense to determine the cause. In 90 % of cases, there is a simple reason for lack of sleep and for this reason it is not a good idea to take sleeping medicine to quickly solve the problem.  
Lack of sleep can be caused by a number of factors:
Lack of knowledge.  Complaints about lack of sleep are often due to a lack of knowledge about individual sleeping needs and the normal phases of sleep. Sleeping needs are individual, which is why five hours' sleep for one person can be just as good as nine hours' sleep for another. 
You should also bear in mind that sleep is divided up into several phases, where in particular the late-night REM phase is concluded by a period of light sleep. This does not constitute a sleeping problem and should therefore not be made into one. 
Outside disturbances. The sleeping environment needs to be dark, quiet and have a pleasant temperature (18-19°)
Lack of physical activity. Exercise results in natural fatigue, which promotes a good night's sleep. Physical exercise, however, causes blood pressure, pulse and temperature to rise and all three must return to their normal levels prior to sleep, which is why it is advisable to exercise three to four hours before going to bed. 
Change in circadian rhythm. Shift work can give rise to sleeping problems, as some people find it difficult to sleep during the day. This will result in poorer responsiveness and productivity, which is why companies have to accept that not all employees are cut out for shift work. Light therapy may help to adjust the inner clock to the new circadian rhythm, whereas sleeping medicine or stimulants are not advised and can lead to drug abuse. 
Stress and crises. In such cases, a minor dose of sleeping medicine may help in the short term. However, in the long term, it is recommended to find a solution to stress-related symptoms.
Physical illness. In connection with depression, anxiety disorders or psychotic states, medical treatment may be the only course of action.



Jet lag

After a six-hour flight to New York, the body's inner clock will feel that it is 2 o'clock in the morning, even though the time is actually 8 o'clock in the evening in New York. This is why it is not a good idea to plan important evening meetings, as your body will feel it is night time and be tired. 
Our inner clock has a tendency to make the day longer than it actually is, which is why jetlag symptoms disappear within a few days. In the meantime, be sure to get plenty of light towards the end of the day, so your inner clock knows that it is still day. You can also prepare for your journey by going later to bed and sleeping late. 
On journeys travelling in the opposite direction, the inner clock will be roughly six hours behind and feel it is the middle of the afternoon when you land in the evening. You can trick the body into believing it is night by taking melatonin.




One out of every 200 people suffers from sudden bouts of sleep lasting up to 20 minutes, between two and six times a day. Such bouts of sleep can occur in any situation and are thus not only a burden but potentially extremely dangerous. 
Symptoms are heavy drowsiness during the day, sudden, brief bouts of muscle paralysis, hallucinations when falling asleep and sleep paralysis, i.e. the sufferer wakes up to discover they are unable to move. 
Narcolepsy can be medically treated.



Power Nap

Some people may take a nap for a few seconds and feel refreshed and rested for the rest of the day, while others may require as much as 15 minutes. 
The need for a power nap in the afternoon varies from person to person, but the introduction of an afternoon break at work is nonetheless a good idea. If you are unable to sleep, time can be spent meditating or used for light exercise.



REM sleep

REM sleep is a state of consciousness in which the body experiences intense activity characterised by irregular, heavy breathing, lower body temperature, higher heart rhythm and lateral eye movement throughout the course of the night. Dreaming occurs during 80% of the REM phase, corresponding to a nightly average of four hours.
Following each REM phase, the body sinks into a deep sleep in order to recharge its batteries. During this period of deep sleep the body produces many of the hormones that combat stress and illness, and it is during this same phase that the body's "frontline of defence" - the powerful killer cells – are renewed, thus strengthening the body's immune system.



Sleep and exercise

Exercise results in natural fatigue, which promotes a good night's sleep. Physical exercise, however, causes blood pressure, pulse and temperature to rise and all three must return to their normal levels prior to sleep, which is why it is advisable to exercise three to four hours before bed.



Sleep needs

To this day, no one is quite sure why we sleep - only that we must sleep in order to survive! 

The need for sleep varies from person to person and changes throughout a person's life. 
Sleep's biological rhythm is probably genetically determined, so it can prove an advantage to know precisely what one's sleep needs are. This can be determined through a system of trial and error; by sleeping for a certain number of hours at night and testing how one's energy level is the following day.  For it is during the day that you must feel the benefit of correct, healthy sleep.
There are, however, certain guidelines for the average number of hours we should sleep throughout our lives. Infants have the greatest need for sleep and sleep between 17 and 23 hours a day. After 8 months, the need for sleep drops and by the age of 12, the child experiences the most dramatic drop in the need for sleep.
Children have a very stable circadian rhythm and an average sleep need of about 9-10 hours a night. Research has shown that children experience a lot of deep, rejuvenating sleep, which enhances the child's ability to concentrate and promotes physical well-being.
The child also experiences a lot of REM sleep which sharpens the mind, the senses and promotes learning. Of the child's 16 hours of sleep, eight are spent in the REM phase in contrast to that of the adult, which on average represents one fifth of sleep.
From the age of 20, most people's need for sleep is between 7 and 8 hours.



Sleep walking

Sleep walking is particularly common in children between the age of four and eight and the phenomenon tends to disappear with age. These nocturnal sorties are quite harmless, but parents must, of course, be aware of any inherent dangers such as open windows and stairs.



Sleeping medicine

Generally speaking, sleeping medicine should only be used for shorter periods (a few days or weeks) or taken in individual doses in particularly stressful situations. 
More specifically, it may be necessary to take sleeping medicine in the following instances:
Physical pain
Both during illness treatment and in connection with incurable illnesses. 
Depression, psychotic states and anxiety disorders
Taken in conjunction with the treatment of the above.
Acute crises and the like



Sleeping advices

  1. Sleep is not just something to put behind you, but should be enjoyed.
  2. Set aside time for sleep and go to bed in good time. 
  3. You should sleep in suitable surroundings. The bedroom should be cool, dark, and quiet.
  4. The bedroom should only be used for sleep and sex. Wherever possible, do not use it as a work room or a TV room, as this will distract from sleep. 
  5. Use the last hour before bed for relaxation, e.g. TV or reading. 
  6. Avoid drinking coffee, tea and Coca-Cola in the evening. Ensure that any alcohol is out of the system, as alcohol results in a more superficial sleep despite having an immediate relaxing effect. 
  7. Try to keep to a regular circadian rhythm, i.e. go to bed and get up at set times.
  8. Devote at least half an hour a day to exercise, preferably three to four hours before bed, as pulse and temperature need time to return to their normal levels.
  9. Avoid being hungry or overly full before going to bed.
  10. Wherever possible, avoid taking sleeping medicine, which must only be used in particularly stressful situations or in connection with psychological or physical illnesses.
  11. Respect the fact that as you grow older, you tend to wake up more often during the night and try not to make this into a problem.
  12. Use your common sense in connection with insomnia and follow the advice on this page. If you continue to experience sleeping problems, contact your doctor.



A little less than half the population aged between 40 and 70 snore, especially men. Snoring is quite harmless, even though it can be a nuisance for the person's sleeping partner. 
Snoring is caused when a slight vacuum occurs in the chest and thus in the airways when inhaling. This is a problem at night when the throat muscles relax and the tongue falls back on itself.  There is now less room for air to pass, which is why vibrations occur and thus snoring. 
Obesity, fatigue, smoking, the use of alcohol or sleeping medicine all increase the risk of snoring. The problem can be alleviated through weight loss, quitting smoking, avoiding alcohol and sleeping medicine. Another tip is to avoid sleeping on your back, or to sleep  with a tooth guard. If the problem is particularly severe, surgical intervention may be an appropriate course of action.



Talking in your sleep

Many people utter a single word, something unintelligible or carry on lengthy monologues in their sleep. This is often caused by stress or illness and is forgotten the following morning.



Type A and B personalities

Whether you are a Type A or a Type B personality is governed by your biological clock in the form of nerve cells in a centre next to the hypophysis on the underside of the brain. This centre is also referred to as the hypothalamus and is partly controlled by the amount of light or darkness detected by the eye. The hypothalamus controls the hormone system, body temperature, hunger, thirst, rage and joy - and also determines whether a person is a Type A or a Type B personality.