The sleep and its recording

From ancient times coma and sleep were considered similar processes. Sleep was seen as a pasive state and considered as something similar to a reversible coma: a coma from which you can be awaken by external stimulus.

The discovery of EEG deeply changed this view. Brain activity during sleep was radically different from the activity recorded in wakefulness, and it was also very different from the activity recorded in pathological states such as coma. A whole set of patterns (spindles, K complexes, slow waves...) not present during wakefulness or comma appeared during sleep. And on the other side wakefulness patterns disappeared during sleep.

Figure 2.1 shows a segment of EEG activity during wakefulness. The so-called alpha rhythm (a continuous activity between 8 and 13 Hz) appears

Figure 2.1: A segment of wakefulness, one second corresponds with two vertical lines. Notice the continuous alpha rhythm

During light sleep (figure 2.2), alpha rhythm disappears and from time to time sleep spindles (a spindle-shaped waveform of limited duration at around 13 Hz) can be seen

Figure 2.2: Sleep spindles (marked in the figure) during light sleep

When sleep becomes deeper, slow waves dominate the record. Figure  2.3 shows the so-called slow sleep in the same time scale than the scale used in the previous figure.

Figure 2.3: A segment of slow wave sleep

Notice that we are speaking of frequencies around 10 Hz (alpha rhythm), 13 Hz (spindle) and 2 Hz (slow waves). All of them have in common that they cannot be heard. Human hearing covers a range of 20-20000 Hz. Imagine that we reproduce the signal connected to a loudspeaker and accelerated 400 times. Alpha rhythm at 10 Hz becomes a continuous pitch around 4000 Hz, sleep spindles at 13 Hz become intermittent tones around 5200 Hz and slow waves under 2 Hz become sounds at around 800 Hz. In this way the whole range of sleep frequencies can be heard.

Now let's describe a really nice feature of sleep. In the long term (hours), sleep has a very well defined structure. Every 90 minutes the sleep becomes deeper and lighter. This is called a cycle of sleep.

An even more surprising feature became apparent from the sleep recording. In each cycle of sleep, a different state called REM sleep is seen. As everybody knows, this kind of sleep takes its name from the rapid eye movements that can be detected.

The picture is more or less complete. We can assign stages to the different epochs and we can evaluate the macrostructure of the sleep. One method is to show the stages of the different epochs as a time series. In the next figure, the whole sleep has been divided into epochs of 30 seconds. We assign a stage to each epoch (being the wakefulness 0 and the deepest sleep 4) and plot them to get a nice plot called hypnogram. REM sleep is shown in green color. The plot is completely different from the previous ones. The time scale of the previous plots covers 30 seconds, the present plot covers 10 hours. Let's see the result in the figure 2.4

Figure 2.4: The hypnogram and the spectrogram of the same night. In the lower frame, the vertical scale is plotted in Hz and time is expressed in seconds

Simultaneously with the hypnogram of the sleep the figure shows the spectrogram of the EEG. A spectrogram plots the frequencies (the power spectrum) present at each time and is frequently used to analyze sounds. When we look at the lower frame, a light line around 14 Hz can be seen. It disappears during REM sleep and wakefulness. Slow waves appear at the bottom showing a waxing and waning behavior. All this subtle structure could be analyzed by hearing it but unfortunately the hearing system does not hear these frequencies. In the next chapter we will accelerate this signal to hear it. Let's do it.

The recording used in these examples (and some other similar recordings) can be downloaded from the Internet. You do not have to be a clinical neurophysiologist to play with sleep recordings. Thanks to the work of two persons that I deeply admire Bob Kemp and George Moody you have a set of recordings in EDF format (including the hypnogram) ready to be used and gain skill in hearing. It is called The Sleep-EDF database. We are going to use the recording called st7121j0. If you see the header of the figures you will see that we used it to create this examples. They were seen using RASCHlab

If you are interested in sleep recording, you should know "A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subject", the classic manual by Rechschaffen and Kales. You can find the whole text at the site of the American Polysomnographic Technologists. This masterpiece include at least three groups of concepts

• How to record sleep: amplifier settings, position of the electrodes...
• How to identify the elements of sleep (sleep spindles, K-complexes...)
• How to score sleep epochs by hand in practice

The document is extremely clear and well written. Each concept is discussed in depth. For the field of the recording of sleep, it is equivalent to Maxwell's equations for electromagnetism or something like this. The original manual contains a very nice set of figures that are very clarifying. In most sleep laboratories there is a copy (or a copy of a copy of a copy...). When writing this document I do not know any place in the Internet where the figures can be downloaded. Perhaps there is some kind of copyright. The manual was published in 1968 and is a basic element implicit in many other discussions and developments. Would it not be time to make a edition on the Internet worldwide available? If you know about it, please tell me the location and I will include the site here. If you start a campaign to support its free diffusion, I am also interested ;-)