2024-03-18 19:30:00
When we sleep, our brain goes through several distinct sleep phases, characterized by specific brain activity patterns. To better observe them, we generally carry out a sleep examination which allows us to obtain a graphic representation of the evolution of the phases throughout the night: the hypnogram. This valuable tool allows doctors to better assess the quality of their patients’ sleep and to identify possible disorders that require special treatment.
Definition: what is a sleep hypnogram?
As its name suggests, a hypnogram is a graphical representation which reports our sleep activity over a given period. It is usually obtained from polysomnography, a recording technique that measures different physiological variables throughout the night.
It allows to observe the different stages of sleep on a temporal axis : in detail, the hours are indicated on the horizontal axis and the sleep stages are indicated on the vertical axis. In particular, we can observe:
the sleep-wake transition phase (stage N1), which corresponds to a transition between the waking and sleeping state;the light slow-wave sleep phase (stage N2), during which brain activity begins to slow down but you can easily wake up;the deep slow-wave sleep phase (stage N3), the most recuperative phase which allows physical recovery and consolidates our memory;and the paradoxical sleep phase (REM)during which brain activity resumes although the muscles still remain asleep.Hypnogram of a young adult © National Institute of Sleep and Vigilance (INSV)
Good to know: each sleep cycle lasts around 90 minutes and adults generally have three to five successive cycles per night. And as the Institute of Sleep and Vigilance (INSV) reminds us, the more the night progresses, the more the composition of our cycles evolves : deep slow-wave sleep, very abundant at the start of the night, becomes increasingly rare, until it disappears completely in the early morning. Conversely, paradoxical sleep, which is brief at the start of the night, will occupy an increasing place in each sleep cycle as the night goes on (source 1).
Indications: what is a hypnogram used for in children or adults?
A hypnogram can be useful in children and adults for several indications:
It can help to diagnose different sleep disorders, such as sleep apnea, restless legs syndrome, or circadian rhythm disorders. By examining the characteristics of the hypnogram, doctors can indeed identify abnormalities in sleep structure that might indicate an underlying disorder. For example, a person with sleep apnea may experience frequent interruptions in breathing during the night, which will result in specific patterns on the hypnogram. Doctors may also assess sleep qualityof their patients by examining the distribution of the different stages of sleep on the hypnogram. Please note: Quality sleep is characterized by smooth transitions between different sleep stages and an adequate proportion of deep and REM sleep. For people with sleep disorders, such as insomnia or sleep apnea, the hypnogram can be used to monitor the effectiveness of prescribed treatmentssuch as behavioral therapies, medications or respiratory support devices.
Instructions for use: how is polysomnography performed to obtain a hypnogram?
As indicated above, the recording of sleep data takes place over an entire night during polysomnography. This reference examination allows to record several physiological variables during sleep, including breathing rate, heart rate, blood pressure, brain activity (electroencephalogram), muscle activity (electromyogram), and eye movements.
Concretely, people are received in a sleep laboratory and equipped with electrodes to measure their brain activity, muscle activity, eye movements, heart rate, oxygen saturation, etc. They generally arrive late in the followingnoon, get ready and go to bed as usual, since the exam takes place throughout the night.
As Dr Ghassan Fayad, a doctor specializing in sleep breathing, reminds us in our article dedicated to polysomnography, “to have interpretable data, it is necessary at least 6 hours of recording “. The electrodes are placed on several parts of the body and connected to a small box which continuously records the various physiological data to feed the hypnogram.
In some cases, a video camera can also be used to record patients’ behavior during sleep.
Interpretation: how to properly read a hypnogram and know if it is normal?
Interpreting a hypnogram is not for everyone… Better to leave it to the specialist doctors! They are responsible for analyzing the transitions between the different stages of sleep, the duration of the cycles, their composition and any interruptions.
Please note: the interpretation of the hypnogram also takes into account the personal context of each person, such as their age, their general state of health and their sleeping habits (in particular their sleeping position). In addition, other monitoring methods can help detect sleep disorders, such assleep diarythe test de somnolence d’Epworth or even the Berlin questionnaire. To find out more, visit our dedicated articles!
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