the individual progresses from infancy, through adulthood to becoming elderly. These changes are dynamic and distinctly related to sleep onset, sleep maintenance, and the amount of time for each sleep stage.
Stages of Sleep
The determination of the sleep stage is dependent on brain wave activity or the electroencephalography (EEG) that is recorded during a sleep study, typically at an attended level I laboratory‐based study or polysomnography (PSG), recorded with surface electrodes over the skull. This will distinguish NREM from REM as well as the sleep stage during NREM sleep. NREM and REM sleep occur in varying proportions during sleep, and they also alternate in a cyclical fashion (Figure 2.1).
The typical pattern in normal sleep is for the individual to progress from wakefulness to the NREM sleep state, followed by the REM sleep state, and then alternating between REM and NREM stages. Overall, a night of sleep comprises about 75–80% of NREM sleep and 20–25% of REM sleep. A disturbance or alteration in the distribution of the sleep staging is typically associated with one of the various sleep disorders.
Figure 2.1 Sleep EEG.
NREM Sleep
NREM sleep initially was broken down into four distinct stages based on EEG activity: Stage 1, Stage 2, Stage 3, and Stage 4. In 2007, a revised method of staging was adopted and NREM Stages 3 and 4 are now viewed as one stage [4]. The stages of NREM are now defined as follows:
Stage N1
This is the stage that transitions from wakefulness and REM to NREM. Typically, it comprises about 5% of the total sleep time. It is considered as a state of drowsiness or light sleep. This stage reflects a change in brain wave activity from rhythmic alpha waves to mixed‐frequency waves, as the individual passes from wakefulness to the initiation of sleep.
Stage N2
This is a deeper stage of sleep but is also considered to be light sleep where the individual maintains a variable level of EEG activity. In this stage, an individual may be aroused easily or awakened. This stage comprises about 50% of the total sleep time and is considered as the first true stage of sleep [5]. The unique and significant EEG features of this stage of sleep are the presence of the K complex and sleep spindles. Sleep spindles are known to act as a filter that blocks out any noxious stimuli, such as noise, light, and even pain that might act to disrupt sleep. Of particular interest to the dentist is that sleep bruxism occurs primarily during this stage of sleep. The K complex may appear as a result of some type of stimulation such as noise or may appear spontaneously. They may also be seen when respiration is interrupted especially during inspiration. The main function is to suppress cortical arousal when there is stimulation and to aid in memory consolidation.
Stage N3
N3 sleep is what previously was known as NREM Stages 3 and 4. This has a unique and individually recognized EEG waveform known as delta waves, hence the term delta sleep, also referred to as slow wave sleep (SWS), deep sleep, or restorative sleep. This comprises about 20–25% of the total sleep time.
REM Sleep (Stage R)
REM sleep is also referred to as dream sleep. Even though it comprises about 20–25% of the total sleep time, this state recurs several times throughout the overall cyclical activity of NREM and REM states during sleep. In normal sleep, each subsequent recurring REM period is typically longer than the prior REM period.
EEG activity is increased with a characteristic “sawtooth” waveform and may appear similar to wakefulness, hence the term paradoxical sleep. There is typically an increase in heart rate, respiration, blood pressure, and eye movements. During this state of increased cerebral activity there may be an immobility or paralysis of the muscles in the limbs, which has been thought to be a mechanism to prevent one from acting out their dreams during sleep [6].
REM sleep may also be contributory to memory consolidation [7], known as procedural and spatial memory. Procedural or declarative memory is associated with automatic retrieval of everyday tasks, facts, and events and spatial memory deals with the ability to have ongoing familiarity with the environment in which we live. Memory primarily involves an area known as the hippocampus, part of the limbic system of the brain. The limbic system, which is not a distinct structure but comprised of various other structures, also involves the hypothalamus along with the amygdala, a structure involved with memory and decision‐making. In addition, two paired structures, the mammillary bodies, also part of this system, are involved with memory [8].
REM sleep consists of two phases, tonic and phasic [9]. Typically a sleep study report will not make a distinction between these two phases. Tonic REM is a unique phase by virtue of the following characteristics:
Atonia (loss of muscle tone) of the skeletal muscles that appears near paralysis except for the diaphragm and extraocular eye muscles.
Desynchronized EEG activity with widespread neural activation or wake‐like EEG activity.
Phasic REM is unique because it may be sporadic as opposed to being continuous and has the following characteristics:
Bursts of REM (eye movements in all directions)
Transient swings in blood pressure and heart rate along with tongue movement and irregular respiration
Myoclonus (muscular jerks), twitching of the chin, and limb movements
REM sleep is typically the stage of sleep associated with dreaming although dreams do occur in NREM, primarily in Stage N2. Dreaming during REM tends to be associated with better recall, as compared to N2 sleep, and have a more defined storyline [10]. REM dreams may also be hallucinatory.
Cycles and Hours of Sleep
During normal sleep the human cycles through the NREM and REM sleep stages four to six times a night. In the adult, regardless of the age, these stages occur at about 90‐minute intervals. In children, these stages are shorter occurring at about 50–60‐minute intervals. In addition, children have different proportions of REM and NREM sleep as well as the number of hours of sleep. A newborn typically sleeps 17–18 hours and 50% of this is REM sleep but by age 3 REM sleep diminishes and is now similar to adult levels [11]. At birth SWS is not present. It is first recognized between two and six months of age. SWS is typically more pronounced in young children, mainly because this is when growth hormone is secreted, appears to decrease after adolescence, and over time continues to diminish (Figure 2.2).
Typically after the age of 70 SWS is reduced and in some cases appears nearly nonexistent. In addition, the elderly may spend more time in bed and less time actively sleeping, also experience more sleep disruption, and there may be an increase in lighter stages of sleep, especially N1 sleep. An added consideration is that the reduction in SWS may be impacted by health‐related issues as well as the use of medications that may impact sleep. It has been reported that the reduced amount of SWS seen in the elderly may be associated
