The earliest mention about sleep also having stages, can be found in the Panchratra Agamas, in the Hindu mythology, which were composed sometime between 3000 BC and 800 AD. They describe two stages of sleep: Sushupti, the stage of deep sleep in which the individual is self-oblivion, unaware as the mind is also at rest along with the five senses and Swapna, the dreaming stage in which the individual enjoys the five objects of senses while all the five sense organs are at rest and only the mind is working. Insomnia and poppy seeds (opium) as its treatment, finds mention in the ancient Egyptian texts. Hippocrates has made a reference to disordered sleep and dreams. However, sleep was considered a passive process and wakefulness an active state until the scientific discoveries of early 20^th century. The development of electroencephalography in 1930s enabled scientists to study the characteristic high amplitude slow waves during sleep and led them to question the basic view that sleep is a passive process.

The modern day knowledge of sleep has evolved, largely, over the last 3–4 decades. Sleep disorders can be classified into: insomnia, parasomnia, hypersomnia, and circadian rhythm disorders. While insomnia refers to lack of sleep, parasomnia refers to abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep. Sleep walking (somnambulism), sleep terror (night terror), sleep sex (sexsomnia), bruxism, sleep related eating disorder, restless leg syndrome, and periodic limb movement, and REM sleep behavior disorder are the common examples of parasomnias. Hypersomnia, as the name suggests, refers to excessive sleepiness and is most commonly caused by fragmented sleep while insufficient sleep and excessive sleep drive are the other causes. Fragmented sleep often results from an abnormal respiratory pattern and this group of disorders is collectively referred to as the sleep related breathing disorders or sleep disordered breathing (SDB).

The current manuscript focuses on this group of disorders as they are now known to be widely prevalent in the general population and have been identified to have far reaching adverse consequences on the cardiovascular and metabolic systems and the neurocognitive and renal functions of the individual. Sleep disordered breathing also has major socioeconomic consequences for the individual as well as for the society at large. The main objective is to provide a 14detailed description of the various aspects of SDB in a simple and comprehensible format which can be easily mastered by a novice and an expert, alike.

Time has come when sleep medicine should be included in the undergraduate medical curriculum and sleep history be included in every patient record.

At the outset, I would like to express my sincere gratitude towards Jaypee Brothers Medical Publishers (P) Ltd. for providing me this unique opportunity of compiling this book. Many thanks to my co-editor, Dr Shivani Swami, for working tirelessly on the manuscripts. I am grateful to all the contributors for having taken out time from their busy schedules to pen down their respective chapters. I owe it to my family and friends, who stood by me throughout the long drawn period of compiling this book.

My main inspiration for dissemination of knowledge comes from my deceased mother, Kusum Nangia and my guide and guru, Professor Premendra Bahadur.