The hypnogram is an infographic – a pictorial representation of the stages of sleep. It is useful for explaining and demonstrating the stages of sleep a person goes through every night. Hypnograms can help show differences in populations (how the elderly sleep differently from the young, for instance). A hypnogram can also be generated during a sleep study as a result of polysomnographic screening. It can be part of an interpretation of the patient’s sleep patterns and in diagnosis of sleep disorders. While hynograms are not unique in the way that a fingerprint are, they are a dynamic and valuable biometric assessment of an individual.
By convention, the hypnogram is horizontal, with time on the x-axis, running from sleep onset on the left to wake-up time on the right. A night’s sleep is shown – about 8 hours.
The vertical, or y-axis, shows stage of sleep. The lower numbers are higher up with stage 0 (waking) at the top. The next tick below is REM sleep. Below that is stage 1, then stage 2, then stage 3 (and sometimes stage 4). There is no quantity on the y-axis; the hypnogram is not formally a graph. The height of the line represents which stage the brain is in at any time, but not any single quantity. The fact that the higher number stages are lower on the diagram is coincidental with the higher number stages being considered “deeper” sleep (e.g. stage 3 is deeper than stage 2, stage 2 is deeper than stage 1). REM is high and between waking and stage 1. REM is also more similar to waking than any of the other stages in an EEG reading, but the placement of the REM is by convention. It could also be at the bottom of the graph, but the tendency of people to wake up shortly before or after REM is another reason to put the REM indicator at the top.
The line running left to right shows the progression of the brain through the stages of sleep. The scale is consistent on this horizontal, x-axis. An hour of time at the beginning of the night is the same length of hypnogram line as an hour later in the night.
The hynogram is not a stand-alone diagnostic tool, and no sleep professional would call out a sleep disorder based just on the hypnogram. But in it’s simplicity, ability to convey information, and clarity, the hypnogram offers researchers, medical practitioners, and interested amateurs a valuable tool.
The sleep cycle is right there on the hypogram. You can see it goes about 100 minutes for a whole cycle, and you can see occasional awakenings around the REM period. The hypnograms of young children show a lot of time spent in deep sleep, and the hypnograms of elderly people shows shallow sleep.
Although the hypnogram is not mathematicl (it's not a plot of two variables), there has been an attempt to describe the human hypnogram with a Markov chain model. It uses a combination of deterministric and probabilistic inflencers to simulate the passage of the brain through the various stage of sleep.