Sleep and dreams have been popular throughout time for writers, researchers and physicians alike. However, most of our modern knowledge of sleep medicine was achieved only in the last four decades. There have been several breakthrough discoveries that paved the way to the scientific investigation of sleep (Table 1).
snoring is the most common cause of sleep apnea and anti snoring devices is so popular around the world.
To date, the understanding of why we sleep and the precise sleep control mechanisms of the brain are far from being completely elucidated. Previously, it was believed that sleep is a time of quiescence and
tranquillity, a time when the body and mind relax to recuperate from the day’s activity, a time when relatively little happens. These assumptions are partially incorrect because sleep is, in fact, an active process. At the start of the nineteenth century, the major sleep theory was that of the hypnotoxins. This theory posited that when we are awake there is an accumulation of poisonous hypnotoxin which drives sleepiness. Hypnotoxins were thought to be detoxified only during sleep. The discovery that serum from sleep-deprived dogs injected into alert dogs caused them to fall asleep (Legendre and Pieron) provided strong support for this theory.
Currently, several mediators, such as adenosine, interleukins, tumournecrosing factor, prostaglandins, lipopolysaccharides and δ-producing proteins, have been proposed to mediate the homoeostatic drive for sleep. Sleep, however, is not regulated by just homoeostatic principles. The discovery by Kleitman that, even with on-going sleep deprivation, one can be less sleepy the following morning suggested that additional factors control the drive for sleep. Indeed, the current agreement among sleep researchers is that sleep is regulated by two factors: the duration of wakefulness (homoeostatic drive to sleep) and the time of day (circadian drive to sleep). The absolute drive to sleep at any point in time is therefore the combination of these two drives.
The discovery of the electroencephalogram in 1928 by Berger provided a quantum leap for sleep research. Applying the new methods to measure EEG activity in sleeping people, or animals, revealed that the transition from wakefulness to sleep is accompanied by specific and well-characterized changes in brain wave activity (Table 2 and Figure 1). Electrocephalography (EEG) has allowed widespread investigations of brain mechanisms controlling sleep and wakefulness by several investgators, including Frederick Bremer, Moruzzi and Magoun, Michele Jouvet and others. There is still an on-going effort for further understanding of the brain circuitry participating in sleep regulation.
different stages: stage 1 (light sleep), stage 2 (consolidated sleep), and stages 3 and 4 (deep, or slow wave sleep). Division of sleep into these stages relies on three physiological variables: EEG, electromyography (EMG) and electro-oculography (EOG) as demonstrated in Table 2 and Figure 1.
The different EEG patterns that are characteristic of non-REM sleep stages are shown in Figure 1. Stage 1 is characterized by relatively low-amplitude θ activity intermixed with episodes of a activity. In stage 2 there are K-complexes (marked with an arrow) and sleep spindles (marked by underlining), whereas stages 3 and 4 are dominated by increasing amounts of slow-wave high-amplitude (δ) activity.
During normal sleep, these stages tend to occur in succession, forming a unique ‘sleep architecture’ (see Chapter 2). Generally, from wakefulness an individual falls into stage 1 sleep, followed by stages 2, 3 and 4 and REM sleep. This succession of sleep stages, culminating in REM sleep, forms a ‘sleep cycle’. The length and content of sleep cycles change throughout the night as well as with age. The relative percentage of deep sleep is highest in the first sleep cycle and decreases as the night progresses, whereas the relative length of REM sleep episodes increases throughout the course of the night. When totalling the various sleep stages through the night in normal young adults, stage 1 occupies up to 5% of the night, stage 2, 50%, and REM sleep and slow wave sleep (SWS) 20-25% each. These relative percentages change with age, as does the cycle length. In infants the normal cycle of sleep lasts about an hour, whereas in adults it lasts about 1.5 hours. Table 3 demonstrates the percentages of different sleep stages and sleep length at different ages.
Brief body movements, which may be accompanied by arousals, mark transitions to and from REM sleep. These four to eight brief awakenings, which are too short to be registered in the memory, are not considered abnormal or sleep disruption. This point is important to keep in mind when dealing with complaints about sleep. It is the difficulty in falling back to sleep, once brief awakening has occurred, rather than the awakenings themselves that may need to be treated. In some sleep disturbances, however, there is a large increase in the number of brief arousals from sleep, which indeed needs medical attention.
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