If we are going to try and understand sleep better then we have got to be able to make measurements. There are four common methods of sleep measurement. First, each can say in the morning how many hours they believe they must have slept or how good a quality of sleep they feel they had.
Then if they are in hospital a nurse can come along every half an hour and make a note of whether she thinks they are awake or asleep (but you may ask will she wake them up by giving such a lot of attention).
We move less when we are asleep and so another method of measurement is provided by the number of movements made or how much energy is put into them.
The fourth and most expensive but most refined approach is to make a record throughout the night of the electrical brain waves and to see for what proportion of time the waves show the size of the sleep.
Methods of sleep measurement
1. Personal estimation
The personal and subjective estimation of how many hours and minutes you have slept is easily enough done but may research investigations have shown that volunteers are not very accurate in estimation the duration of their own sleep to judge by the more exact measurements of how much sleep they got according to the electrical brain-wave records.
In many of the research studies at our laboratory, we have asked people to make an estimate or rating of the equality of sleep they had had during the preceding night, and to do this they filled in each morning what is known as a visual analog scale (see opposite).
The method is a cheap one and is acceptable to most people who are willing to fill in these rating scales every morning for many months.
In the end, the number of millimeters between the left-hand end of the scale and the mark that why has made in the morning can be measured.
The visual analog scale is used by researchers to gauge the quality of volunteer’s sleep. The volunteer makes a mark on the horizontal line, according to how well he feels he has slept.
In one test we gave volunteers pills regularly at bedtime, which, without their knowledge were suddenly switched from inactive pills to identical-looking ones containing one of the more usual sleeping drugs.
The visual analog scale rating at once told us that the volunteers thought they were sleeping better.
When after some month of nightly intake of the pills containing the drugs the pills looking but inactive pills then they felt that they were at first sleeping very badly need and the measurements in millimeters on the visual analog scales at once betrayed this.
2. An observer’s estimation
Similar use can be made of nurse’s judgments. Occasionally people claim that they have hardly slept a wink for many years.
The great majority feel that they are being deprived of something that they wish they could enjoy more of and are concerned about what they feel is a lack of sleep.
We and many other sleep researchers have studied the sleep of such people; and if you share this problem you may be surprised and reassured by what we found.
Simple observation alone such as a nurse can make, invariably confirmed that such people actually are asleep nearly the whole night, perhaps snoring with every appearance of continued slumber and quite unaware of the presence of the observer.
Generally, the more restful your sleep is, the fewer movements you make. On average you change positively about forty to fifty times a night.
3. Recording body movement
It is possible to use a small device rather like one of those wristwatches that are self-winding and in which a weight is moved in position whenever the wearers arm moves to make a record of the body’s activity.
The movement of the weight alters a counter and so gives, over a period of time, a rough indication of the number and vigor of movements.
Such measurements of body movements are not very reliable for telling us the point at which somebody feels asleep because obviously you can be awake and lie absolutely still for quite a long period.
Some people move frequently while they are asleep and others rarely so that the method is not very precise for telling us whether any one person has been asleep during any particular period of the night.
Taking the measurements for a group of people altogether, however, the method gives a good general indication of the most restful times.
4. The electroencephalograph (EEG)
Electrical brain waves can be measured because the brain operates by chemical and electrical means. It is made up of tiny units called cells, surrounded by a fluid; and the chemical constitution within the cells differs from the chemicals in the surrounding fluid.
Just as a battery that we put into a torch will produce small electric currents because of the chemical it contains so the cells in the brain generate tiny electrical currents.
Over the head as a whole, the electricity from the millions of cells in the brain is joined together.
If therefore small silver discs are fixed to the scalp and wires are then lead from those discs into a sensitive’s machine for measuring electricity the machine can show us the variations in how the cells are working in the form of nonelectrical brain waves or the electroencephalogram.
The machine that we use, called an electroencephalograph (EEG), is really a very delicate and expensive voltmeter.
Instead of having a needle on a scale, ink flows from the tips of fine pens onto paper, so that when the needles-pens make movements the electrical brain waves are written out as ink traces upon moving paper.
If we arrange for there to be an endless supply of paper, made up of individual pages separated by perforations, and if each of those pages takes twenty seconds, turn through, then we can say how many pages (thus minutes) of paper flowed during the time between lights-out and first falling asleep.
Again, by studying the shapes and patterns of the waves we can say how much time was spent in wakefulness, and in which parts of the night it occurred; and we can also say how many minutes were spent in which stages of sleep.
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