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T. Penzel et al.
sleep disorders as well. Cyclical variations of heart rate combined with respiration-modulated changes in ECG morphology (amplitude of R wave and T wave)
provides reliable detection of sleep-disordered breathing. Conclusions: The assessment of signals being easily accessible like ECG and heart rate can help to assess
sleep, sleep stages and sleep disorders with an acceptable accuracy, even if reflecting
physiological functions indirectly.
Keywords Sleep stages · Autonomic function · Heart rate variability · ECG ·
Cardiovascular regulation · Sleep apnea · Cardiorespiratory coupling
20.1 Introduction
Sleep takes about one third of human life and is important for rest and for recovery
from daytime activities. Sleep is not a homogenous state but comprises different
stages such as light sleep, deep sleep and rapid eye movement (REM) sleep, which
serve different physiological functions. For example, deep sleep seems to be most
important for physical recovery and for removing waste metabolites from the brain
[11]. Growth hormone is secreted during deep sleep, and this sleep stage is most
important for immune system build-up. REM sleep is associated with dreaming
and memory functions, and thus REM sleep is thought to be important for mental
recreation and mood. During the process of sleep, these different sleep stages follow
one after the other in a well-defined order and sequence starting with light sleep,
followed by deep sleep and then REM sleep. This sequence is called a sleep cycle
and has a duration of 80 to 110 min in general. Over the course of the night, there
are between 4 and 6 such sleep cycles. In the beginning of the night a sleep cycle
contains much more deep sleep and very short REM sleep, while towards the end
of the night, a sleep cycle has little, if any deep sleep and much longer REM sleep.
As such, the beginning of sleep and the end of sleep are very different. The entire
‘sleep cycle program’ is sensitive to behavior, external influences (e.g., light, noise,
temperature), internal influences (e.g., food, stress, drugs, medications), and is much
influenced by disease. Of particular importance in this context are sleep disorders
that originate out of the sleep process itself. Sleep medicine has defined 66 distinct
sleep disorders, among which insomnia (the inability to fall or stay asleep) and
sleep apnea (respiratory cessations during sleep) are the two most prevalent and
well-known examples.
In order to investigate sleep, in healthy subjects and in patients with sleep
disorders, polysomnography (PSG) is commonly used in a sleep laboratory. PSG
recordings include several electroencephalography (EEG) electrodes placed on welldefined locations on the scalp. To recognize REM sleep and to distinguish the
different sleep stages, in addition to EEG, the electrooculogram (EOG) and the
electromyogram (EMG) of the chin are recorded. A screen shot of these signals
is depicted in Fig. 20.1a. In order to detect sleep disorders, such as sleep apnea,
additional signals need to be recorded. At first, cardiac and respiratory signals were
T. Penzel et al.
sleep disorders as well. Cyclical variations of heart rate combined with respiration-modulated changes in ECG morphology (amplitude of R wave and T wave)
provides reliable detection of sleep-disordered breathing. Conclusions: The assessment of signals being easily accessible like ECG and heart rate can help to assess
sleep, sleep stages and sleep disorders with an acceptable accuracy, even if reflecting
physiological functions indirectly.
Keywords Sleep stages · Autonomic function · Heart rate variability · ECG ·
Cardiovascular regulation · Sleep apnea · Cardiorespiratory coupling
20.1 Introduction
Sleep takes about one third of human life and is important for rest and for recovery
from daytime activities. Sleep is not a homogenous state but comprises different
stages such as light sleep, deep sleep and rapid eye movement (REM) sleep, which
serve different physiological functions. For example, deep sleep seems to be most
important for physical recovery and for removing waste metabolites from the brain
[11]. Growth hormone is secreted during deep sleep, and this sleep stage is most
important for immune system build-up. REM sleep is associated with dreaming
and memory functions, and thus REM sleep is thought to be important for mental
recreation and mood. During the process of sleep, these different sleep stages follow
one after the other in a well-defined order and sequence starting with light sleep,
followed by deep sleep and then REM sleep. This sequence is called a sleep cycle
and has a duration of 80 to 110 min in general. Over the course of the night, there
are between 4 and 6 such sleep cycles. In the beginning of the night a sleep cycle
contains much more deep sleep and very short REM sleep, while towards the end
of the night, a sleep cycle has little, if any deep sleep and much longer REM sleep.
As such, the beginning of sleep and the end of sleep are very different. The entire
‘sleep cycle program’ is sensitive to behavior, external influences (e.g., light, noise,
temperature), internal influences (e.g., food, stress, drugs, medications), and is much
influenced by disease. Of particular importance in this context are sleep disorders
that originate out of the sleep process itself. Sleep medicine has defined 66 distinct
sleep disorders, among which insomnia (the inability to fall or stay asleep) and
sleep apnea (respiratory cessations during sleep) are the two most prevalent and
well-known examples.
In order to investigate sleep, in healthy subjects and in patients with sleep
disorders, polysomnography (PSG) is commonly used in a sleep laboratory. PSG
recordings include several electroencephalography (EEG) electrodes placed on welldefined locations on the scalp. To recognize REM sleep and to distinguish the
different sleep stages, in addition to EEG, the electrooculogram (EOG) and the
electromyogram (EMG) of the chin are recorded. A screen shot of these signals
is depicted in Fig. 20.1a. In order to detect sleep disorders, such as sleep apnea,
additional signals need to be recorded. At first, cardiac and respiratory signals were