Sleep is a very basic human need that appears to be a fundamental process. Yet the mechanism and reason why we sleep is extensively researched and yet still not fully understood. Sleep is characterized by 3 features:
Some believe it to be habitual, as many of us have different sleep requirements and different sleep patterns / habit or cycles. Sleep is also seen as state of unconsciousness in which the brain is relatively more responsive to internal than to external stimuli. The predictable cycling of sleep and the reversal of relative external unresponsiveness are features that assist in distinguishing sleep from other states of unconsciousness. The brain gradually becomes less responsive to visual, auditory, and other environmental stimuli during the transition from wake to sleep.
It used to be believed that humans have to sleep for the body to get rest; however, we are currently aware that sleep is meant for the brain to get rejuvenation and rest.
The average human spends about 6 to 8 hours per day sleeping, for some it can be up to 10 hours per day, while others can do with just 4 hours per day. Hence, we spend about one third of the day sleeping, and therefore, one third of our life time sleeping, yet so many of us take sleep for granted.
As we spend one third of our life sleeping, for the average human who lives up to about 80 years old (current life expectancy), sleeps a total of 26 years of his life!
Dolphins are intelligent mammals, they are able to rest half their brain and keep the other side of the brain awake while swimming and keeping watch. Lions snooze about 12 hours per day, and they precisely do so because they are the top of the food chain (all other animals dare not disturb the lion). Elephants only sleep about 4 hours per day, and they do so while standing up-right. While we might know of some people who can do just that, most people would find it difficult to sleep upside down like the bat would for 3 months during winter (hibernation).
Most of us assume that sleep is a passive event, where we close our eyes and all our bodily functions “turn / switch off”. Contrary to popular belief, sleep is an active process involving complex interactions with increased metabolic functions and activities within the brain. Scientists have shown that during dream sleep, the brain activity is more intense compared to the brain in non-dream sleep and in the awake state!
Sleep is divided into 2 main phrases: dream sleep (rapid eye movement / REM) and non-dream sleep (non-REM). The dream sleep is also known as slow-wave/delta wave sleep (SWS) when the brain waves slow down to a very slow and coordinated pattern. On average, dream sleep comprises about 25% to 30% of the entire sleep in an adult. In a child however, dream sleep may be as high as 50% of the entire night’s rest. Research done on metabolic activity of the brain during different phrases of sleep and wake states, demonstrated increased metabolic activity in most parts of the brain during dream sleep compared to non-dream sleep and even the wake state. Most sleep experts believe that dream sleep is crucial for the human.
It is well accepted that dream sleep is the most important element in the sleep process, for well being, memory re-building, rejuvenation and mental alertness. Hence, dream sleep is vital to the human mind and body.
Dream sleep is characterized by slow delta waves in the brain, increased metabolic activity in the brain, and rapid eye movements (REM). Rapid eye movements during dream sleep are a medical phenomenon that helps us identify dreaming events but scientist cannot explain their need. Simplistically, dream sleep is a highly active brain in a “paralysed” body. This is a perfection made by our divine maker, so as to prevent us from acting out our dreams (i.e. if one was dreaming that one was playing soccer, one does not actually kick and act it out in bed, endangering one’s bed partner). This however, works against the patient with a narrow airway, as the already narrowed airway will lose its tone during dream sleep and collapse, leading to upper airway obstruction, cessation of breathing and lack of oxygen. Therefore, this leads to stress on the heart, brain and other organs in the body. This condition is called obstructive sleep apnoea (OSA).
Obstructive sleep apnoea is the commonest sleep disorder. It results in interrupted sleep, sleep fragmentation and poor sleep quality. This is similar in patho-physiology to, what we so commonly encounter in our daily life, called sleep deprivation. During the day, the patient is excessively sleepy, has poor concentration, poor memory and becomes irritable. Long term health consequences include high blood pressure, heart disease, strokes and sudden death during sleep.
Non-Dream sleep is believed to be a complex interaction between the higher brain centers and the mid-brain. The exact function of this non-dream sleep (also known as non-REM sleep) is not known, although many sleep scientist postulate that during this decreased metabolic demand state in sleep, the brain can now replenish energy glucose / glycogen stores; perhaps to get ready for the next REM phase.
There are 4 stages of non-dream sleep. They progress from stage I, II, III and IV. Stage III and IV are also known as slow wave sleep (SWS).
The circadian (cyclical) sleep rhythm is one of the several intrinsic body rhythms modulated by the hypothalamus (mid-brain). The sleep centre sets the body clock to approximately 24 hours, with both light and dark exposure influences the 24-hour cycle. The light exposure stimulates the sleep control centre directly and modulates the sleeping pattern as well. A practical purpose has been proposed for the circadian rhythm, using the analogy of the brain being somewhat like a battery charging during sleep and discharging during the wake period.
The normal human brain goes through a very systematic process every night during sleep. The brain would go through the 4 stages of non-dream sleep and reach dream sleep in a cyclical fashion. In essence, there are some “rules” that the brain follows:
Stage I sleep – brain transitions from alpha waves (common to people who are awake and having a frequency of 8 to 13 Hz) to theta waves (frequency of 4 to 7 Hz). This is indicative of the brain “slowing” down from one stage to another. This stage is sometimes referred to as somnolence, or “drowsy sleep”. This stage is also associated with the onset of sleep and may have sudden twitches and hypnic jerks (jerking just when a person is about to fall asleep, when your spouse “kicks” you while falling asleep).
Stage II sleep – characterized by “sleep spindles” (12 to 16 Hz) and “K-complexes.” During this stage, muscular activity as measured by electromyography (EMG/muscle activity) lowers and conscious awareness of the external environment disappears. This stage occupies 45 to 55% of total sleep.
Stage III sleep – the delta waves, also called delta rhythms (0.5 to 4 Hz) make up less than 50% of the total wave-patterns. This is considered part of deep or slow-wave sleep (SWS) and appears to function primarily as a transition into stage N4. This is the stage in which night terrors, bedwetting, sleepwalking and sleep-talking occur.
Stage IV sleep – delta-waves make up more than 50% of the wave-patterns. Stages III and IV are the deepest forms of sleep; stage III is effectively a deeper version of stage IV, in which the deep-sleep characteristics, such as delta-waves, are more pronounced
One sleep cycle comprises of four stages and last for about 90-120 minutes. An average sleep period a person will experience 4 to 5 complete sleep cycles. The sleep cycle begins with four stages of SWS (Slow-Wave Sleep), also called NREM (Non-REM). Note that after the completion of the 4th stage, the 5th stage does not immediately begin; instead, the first 4 stages quickly reverse and are then immediately followed by a REM period. The first REM period will occur roughly 90 minutes after falling asleep (REM latency); thus the first REM period will last only about 10 minutes, given the length of each sleep cycle being roughly 100 minutes. The length of the stages is not static, however: as the night proceeds, the length of stages 3 and 4 (also called delta or deep sleep) begins to wane, and the length of REM sleep increases, up to about one hour in length after a number of cycles. Therefore, as the night goes on, you dream for longer periods of time.
Man has never been more sleep deprived then before, especially with our stressful lives and work. It was thought in the old days, that God wanted man to sleep for 12 hours per day, as the sun rose at about 6am plus and set at 6pm plus, hence, man was required to sleep during the night. It was not until 1879, that one man, Thomas Edison, invented the light bulb, this broke mankind’s sleep cycle and currently, we have to work beyond the sun setting at 6pm.
There is no real rule as to how much sleep one must take, but there are general guidelines and needs depending on the age of the person.
|Age||Average amount of sleep per day|
|Newborn||up to 18 hours|
|1-12 months||14-18 hours|
|1-3 years||12-15 hours|
|3-5 years||11-13 hours|
|5-12 years||9-11 hours|
|Adults, including elderly||7-8 (+) hours|
|Pregnant women||8 (+) hours|
The sleep hormone, called melatonin, is sensitive to the dark. It is produced in the pineal gland of the brain and primarily causes drowsiness and enhances deeper and more natural sleep. It is usually produced about 8 to 9pm in the evening and peaks at about midnight to 1am, and dwindles after that. Melatonin is in fine balance with the stress hormone, cortisol. Cortisol is produced in the adrenal glands, and is required for stress. When the cortisol levels are high, the patient may not be able to have restful sleep, resulting in sleeplessness or insomnia. Melatonin may be used as a sleep enhancer to aid patients with sleeplessness; melatonin is also frequently used in people who cross time zones regularly and suffer from jetlag.
Many studies have showed a variety of features consistent in patients with sleeplessness or insomnia, they have:
more night-to-night variability in their sleep.
Managing stress is a very important (and often complex and difficult) issue to treat patients with sleep disorders. Management of stress can range from ventilation therapy, to counseling, to exercise and to relaxation therapy. Decreasing a patient’s stress would lower the blood levels of cortisol, and hence, allow the patient to sleep better.
Dr Kenny Pang, ENT Consultant, Asia Sleep Centre, Singapore.