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Like waking, sleep is an active and dynamic process during which characteristic activities and cyclical patterns have been observed (Carskadon & Dement, 2011). Normal human sleep comprises two well defined states—non-rapid eye movement sleep (NREM)

and rapid eye movement sleep (REM). Associated with each state are particular electroencephalographic (EEG) wave forms, somatic movements, muscle tone changes and respiratory patterns which can be objectively measured using polysomnography (see

Measuring Sleep, page 15). The patterns, timing and absolute and percentage

contributions of the various stages and transitions during sleep are collectively referred to as ‘sleep architecture’ (Redline et al., 2004).

Except for in early infancy, sleep is entered through NREM, which is commonly classified into three stages in healthy adults, N1, N2 and N3, with each stage indicating a deeper level of sleep state (Silber et al., 2007). As sleep deepens, the threshold for arousal as a result of endogenous or exogenous stimuli becomes higher. At the commencement of a normal night’s sleep, N1 sleep usually lasts just a few minutes before transition into N2 which lasts between 10–25 minutes. N3 sleep lasts around 20–40 minutes in the first cycle of night sleep. The deepest stage of sleep, N3, is also referred to as ‘slow wave’ sleep (Novelli, Ferri, & Bruni, 2010).

After a time in NREM sleep, an increase in body movements may be seen as the sleeper ascends from N3 through N2 and N1 into a period of REM sleep. The first period of REM sleep at night is very short (1–5 minutes), but subsequent periods become longer across the night. At the same time, less time is spent in deep sleep as the night progresses. During REM sleep EEG activity is similar to that seen during wakefulness and dreaming occurs mostly in this stage. REM sleep occupies about 20–25% of adult sleep duration, with 75–80% of sleep spent in NREM sleep. A full night’s sleep consists of multiple cycles of REM and NREM, with each full cycle taking, on average, 90–110 minutes to complete.

During waking, the brain is in a heightened state of activity. Neuronal networks throughout the brain communicate in a rapid, asynchronous fashion in response to the constant stream of information being received from the internal and external environments. Evidence of this activity can be observed through analysis of the oscillations in electrical potentials in the brain’s cortex, recorded in EEG monitoring. EEG waves during waking are typically high in frequency and low in amplitude (beta waves in

the 14–30 Hz frequency range). Alpha waves (in the 8–12 Hz range) are dominant during

quiet, eyes-closed, relaxed waking. As sleep onset occurs, cortical firing becomes more synchronous and EEG waves increase in amplitude. In stage N1, the sleeper begins to lose conscious connection with the external environment and theta waves in the 4–7 Hz range become predominant. Loss of connection with the external environment is complete in N2, and specific characteristic waveforms appear as the hallmarks of this stage (‘sleep spindles’ and ‘K-complexes’). As the sleeper moves into deep sleep (N3), large groups of cortical neurons fire in synchrony resulting in EEG waves of high amplitude and slow

frequency. They are known as delta waves and are in the 1–3 Hz range (Fuller, Gooley, &

Saper, 2006). The percentage of time spent in slow wave sleep appears to decline

significantly from pre-pregnancy to the first trimester, with no improvement seen until the postpartum (Lee, Zaffke, & McEnany, 2000).

During REM sleep, EEG waves return to the beta range (high frequency) and, on observation, resemble waking activity. The time taken to reach the first episode of REM during a sleep period, the amount of REM activity in a sleep period and duration in REM, are all altered in depression. Objective studies of sleep structure during pregnancy have produced mixed findings. It appears that when compared to non-pregnant women, the percentage of REM sleep experienced by pregnant women is very similar or may reduce slightly across pregnancy (Balserak & Lee, 2011).

Sleep is not continuous as this text-book style description may imply. Fragmentation is the term used to describe any interruptions to sleep continuity and some degree of fragmentation is a feature of normal sleep. Sleep can become fragmented via a number of mechanisms. These include gross physical disturbance such as being shaken awake by another individual, physiologic responses to other subtle endogenous and exogenous stimuli, such as changes in blood pressure or environmental noises, and as a result of expected transitions between sleep stages (Kato, Montplaisir, & Lavigne, 2004).

Fragmentation can be observed as changes in cortical EEG activity, and defined as arousals or shifts to N1 sleep or wake from any of the other sleep stages (Paruthi & Chervin, 2011). An arousal is an abrupt shift in EEG frequency towards waking EEG, with characteristics including frequencies higher than 16 Hz, without spindles, and with or without theta and alpha activity (Bonnet et al., 2007). The difference between a cortical arousal and an awakening is a somewhat arbitrary and flexible notion, however, in 1992 the American Sleep Disorders Association published criteria by which EEG events in sleep can be defined, scored and classified for clinical and research purposes (Bonnet et al., 1992). Briefly, the definition requires an abrupt shift in EEG frequency towards awakening, which must have been preceded by at least 10 continuous seconds of sleep, and which lasts for at least 3 seconds. Depending on the stage of sleep during which the arousal is observed, further criteria may need to be met, such as changes in muscle tone. Arousals lasting between 3–10 seconds are sometimes referred to as micro-arousals, while arousals lasting longer than 15 seconds may be referred to as awakenings (Kato et al., 2004), although the sleeper may not have any conscious memory of having been fully awakened. All arousals elicit physiological consequences including changes in breathing, heart rate and blood pressure.