If your doctor recommends that you spend a night in a sleep clinic, you may be doing it for the ‘gram. In this case, though, we’re not referring to Instagram, the popular social network, but rather to a hypnogram, a graph that shows how your nightly sleep unfolds.
Hypnograms use several sources of data, including about your brain, eye, and muscle activity, to analyze your sleep cycle and determine how much time you are spending in each sleep stage. That analysis is represented in a chart that offers a straightforward way to visualize your sleep architecture.
This guide provides a detailed explanation of hypnograms. It reviews what they are, how they are generated, how they are interpreted, their limits, and what to do if you think a hypnogram might be of benefit to your nightly sleep.
What Are Hypnograms?
By definition, a hypnogram provides details about your sleep. In particular, it is a graph that provides a visual representation of the amount of time that you spend in each sleep stage during one night. In doing so, it offers a view of your sleep architecture, which is a term used to describe the progress of your sleep cycle over the course of the night.
The horizontal axis (x-axis) of a hypnogram is time, usually measured in hours or increments of 15-30 minutes. The vertical axis (y-axis) charts your sleep stage, including stages I-III in non-REM sleep, REM sleep, and wakefulness. The graph offers a view of how many sleep cycles you went through, how many times you awoke in the night, how much time you spent in each sleep stage, and how often you were transitioning between stages.
Most hypnograms cover a time period of just one night, but it is possible to produce a 24-hour hypnogram when someone has a condition, such as narcolepsy, that may affect sleep throughout the day.
How Are Hypnograms Used?
A hypnogram can provide vital information that helps distinguish between normal and disturbed sleep.
Some data from sleep studies provide important metrics like the overall sleep time, the number of sleep cycles, and the total time spent in each sleep stage. While this data is useful, a hypnogram goes even further by showing with granular detail how each of those stages unfolded. As a result, the hypnogram can show if certain sleep cycles are longer than others or involve more time in REM or NREM sleep.
A hypnogram can illustrate when a person has awakenings during the night, and this can be useful in diagnosing obstructive sleep apnea (OSA). OSA is a type of sleep-disordered breathing in which people have frequent pauses in breathing, called apneas, while sleeping. These apneas can cause awakenings that are often visible on a hypnogram. A study by researchers at Johns Hopkins found that applying advanced statistical analysis to the hypnogram could help identify specific patterns of sleep disturbances in people with OSA.
A one-night or 24-hour hypnogram can play a role in diagnosing narcolepsy. In a one-night study, the hypnogram may show a pattern of entering REM sleep quickly after falling asleep which is more common in people with narcolepsy. A full-day hypnogram can demonstrate the frequent instances of falling asleep that can plague people with this condition.
For anyone with an ongoing sleeping problem, a hypnogram can be a useful tool to review with a doctor or sleep technician to help diagnose and then treat underlying sleep disorders or disturbances.
How Are Hypnograms Produced?
Producing a hypnogram requires first collecting data and then scoring that data and formulating it into a graph.
The primary data that are used to create a hypnogram come from an electroencephalogram (EEG), an electrooculogram (EOG), and an electromyogram (EMG). An EEG tracks the activity of the brain, an EOG tracks eye movement, and an EMG tracks muscle activity. For most hypnograms, data for the EMG comes from monitoring muscle activity under the chin.
Brain, eye, and muscle activity fluctuates while we sleep, and changes detected by these tests can be used to determine what stage of the sleep cycle we’re in at any point in the night. To standardize this analysis, the American Academy of Sleep Medicine (AASM) has created a manual for scoring sleep. These criteria have been clinically validated and provide a common system for categorizing the sleep stages based on data from EEG, EOG, and EMG.
Once sleep scoring is complete, the data can be plotted on a graph, creating a hypnogram that allows the progression of each sleep stage to be seen as a series of peaks and valleys. These graphs can resemble the buildings of a city’s skyline, lending this movement through sleep cycles the name “sleep architecture.”
How Are Hypnograms Interpreted?
Hypnograms should be interpreted in conjunction with a doctor or a sleep specialist who has knowledge of how to read a hypnogram and experience doing so with their patients. A health professional can best explain the significance of the graph and the changes in sleep stages during the night.
For people who want to read their own hypnogram, the basics are relatively straightforward. The far left of the graph is when you first go to sleep, and moving to the right represents time elapsing during the night. The location of the line vertically demonstrates the sleep stage at that time.
When the line is horizontally flat, it indicates continuous time spent in the same stage. When the line moves vertically, it represents a change in your sleep stage. You may also notice spikes in the hypnogram that can show awakenings during the night.
Even though you can read a hypnogram on your own, truly interpreting what it means as far as your sleep and any potential sleep disorders requires collaborating with a doctor. Not only can a doctor go into more detail about the significance of your sleep architecture, but they can also discuss potential treatments for sleeping problems and other steps to improve your sleep.
What Are the Limits and Possible Downsides of a Hypnogram?
A hypnogram is a useful tool for visualizing the nature of your sleep and identifying possible sleep problems, but it’s not perfect. As with virtually any medical assessment, there are some limitations and potential downsides.
- It relies on accurate sleep scoring that is subject to human error. The sleep scoring of your EEG, EOG, and EMG data is done by a person, and as with any human analysis, there is a margin for error. AASM’s standardized criteria and training programs reduce errors and variation between scorers, but there remains some possibility that your hypnogram will reflect an inaccurate calculation of your sleep architecture. Research into applying algorithms and machine learning to analysis of sleep data is ongoing and may yield continuing improvements in sleep scoring.
- It is usually only generated for one night. Most sleep studies gather EEG, EOG, and EMG data from just one night. While this may be sufficient to detect major problems that appear every night (such as most cases of sleep apnea), they may not be representative of how you sleep in general. A hypnogram, then, may not perfectly capture your normal sleep pattern.
- Your may sleep differently in a sleep clinic or when using sleep tracking tools. To get the data for a hypnogram, it’s common to need to spend a night in a sleep clinic. Even if conducted at home, the necessary tests require sleeping while being hooked up to multiple sensors. Being in an unfamiliar place or having these testing devices attached to you may disrupt your sleep and create a variation from how you typically sleep.
- The hypnogram may have smoothed lines for simpler visual presentation. The x-axis of the hypnogram can be broken down into many increments. To simplify the graph, a longer timeframe may be used, and small vertical movements may be smoothed out for visual coherence. For this reason, the hypnogram should not be understood as an exact visualization of your sleep.
How Do I Get a Hypnogram?
To get a hypnogram, you first need to do a sleep study that gathers the necessary data for sleep scoring and analysis of your sleep architecture. This study is almost always ordered by a doctor who can coordinate with you about whether to do the study in your home or in a sleep clinic.
Working with a doctor to order a sleep study can help ensure that the tests are conducted accurately and produce the highest-quality data. Having the test prescribed by your doctor can also help you get insurance coverage for the costs of the test. For these reasons, anyone who has serious or persistent sleeping problems or who is interested in viewing a hypnogram of their sleep should discuss their concerns with a doctor who can then address the potential benefits and risks of a sleep study and hypnogram.