The average human spends about six to eight hours - about a third of a day - sleeping.
Hence, we spend about a third of our life sleeping - 26 years for the average Singaporean who lives to the age of 80 - yet so many of us take sleep for granted.
The mechanism of and reason for sleep are still not fully understood despite extensive research.
It used to be believed that humans have to sleep for the body to get rest. However, we are aware currently that sleep is meant for the brain to rest and be rejuvenated.
This usually occurs during a phase of sleep called dream sleep or rapid eye movement (REM) sleep.
On average, dream sleep comprises about 25to 30 per cent of the entire sleep cycle in an adult, and the rest is non-dream sleep or non-REM sleep.
In a child, dream sleep may be as high as 50per cent of the entire night's rest.
It is well-accepted that dream sleep is the most important element in the sleep process, for well-being, memory rebuilding, rejuvenation and mental alertness. Hence, dream sleep is vital to the human mind and body.
Simplistically, dream sleep results in a highly active brain in a 'paralysed' body.
This prevents us from acting out our dreams, but works against someone with a narrowed airway.
When obstructed, the upper airway - the nose and throat - has a major impact on one's sleep pattern and quality, which in turn affects one's health and quality of life.
Any swelling or obstruction in the nose results in turbulent airflow and breathing difficulty, and more negative pressure is required to inhale air into the lungs.
The higher negative pressure in the throat - a dynamic structure that acts as a conduit to channel air from the external environment into the lungs - creates a vacuum effect and causes the throat muscles to collapse.
Any part of the throat - palate, walls, tonsils and tongue - that becomes enlarged or swollen also obstructs breathing.
These effects are more pronounced during sleep, as one's muscles become completely relaxed and flaccid.
The spectrum of diseases related to reduced airflow through the upper airway during sleep is known as sleep disordered breathing, which includes snoring and obstructive sleep apnoea.
Snoring occurs when turbulent airflow causes soft tissues to vibrate. For example, obese patients often snore because they frequently have a thick and fatty soft palate and throat walls, which vibrate during sleep.
Snoring does not cause patients to stop breathing or feel tired during the day.
Obstructive sleep apnoea, however, causes snoring, breathing pauses, a lack of oxygen and daytime sleepiness.
In Singapore, the incidence of obstructive sleep apnoea is estimated to be about 15 per cent.
The very important sleep component of dream sleep will not be able to take place if the body is deprived of the fundamental basic need for oxygen. This results in interrupted sleep, sleep fragmentation and poor sleep quality.
Common patient complaints include early morning tiredness and morning headaches from the low oxygen levels, and dry mouth and throat in the morning from mouth-breathing and snoring.
During the day, patients are excessively sleepy, have poor concentration and memory and become irritable or moody.
Other symptoms include forgetfulness, depression, irritability and, less commonly, impotence in men.
During the night, patients may complain of frequent awakenings with a choking and gasping sensation, nocturia (frequent passing of urine at night), or nightmares.
Dr Kenny Pang is the president of the Asean Sleep Surgical Society. He invented two surgical techniques to treat snoring and sleep apnoea and has written more than 32 papers in international journals and 12 chapters in American textbooks.
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