Oscillation During Sleep

diurnal variation would be incomplete without special mention of the changes which occur in sleep. The diminution of all activity during sleep is well recognized. What is not so certain, however, is the character of the sleeping period itself. Numerous investigators have been interested in the "rhythm" or "depth" of sleep and have made periodic tests in an attempt to measure it.

Sleep and waking are purely relative terms, and the organism may be asleep to some stimuli and awake to others. A mother who will jump from her bed at the slightest sound from her baby will sleep through a three-alarm fire. In order to satisfy the "general sleep," it would be necessary to measure the effects of the myriads of stimuli playing upon the organism at the moment and to composite them. The fulfilment of these conditions is practically impossible and does not commend the method to further use.

A more valid experimental attack upon the problem of oscillation during sleep is that which measures directly or indirectly the energy expenditure of the subject at different hours of the night. There are a number of possible methods, including oxygen consumption and cardiovascular changes. Related in a more remote way are muscle tension and restlessness. Energy expenditure and cardiovascular functions are depressed during sleep, frequently assuming sub-basal values after five or six hours of sleep. There is considerable evidence, however, that the sleep state per se is not associated with this decrement. For example, changes in blood pressure, oxygen consumption, and cardiac output during sleep are similar to those obtained at the same hour of the night in the waking state under completely relaxed conditions.

We know that muscular relaxation is one of the chief conditions of sleep. The measurement of tension would, therefore, constitute a perfect method for studying the oscillations in sleep. Unfortunately, available measures of tension are relative to an arbitrary base-line, whereas what is required is an absolute score. Presumably tension and restlessness are positively related, and the amount of restlessness or motility exhibited at different portions of the sleep period can be easily determined.

A typical subject about one-third of the total time in bed is marked by stirring every ten minutes or oftener, and the other two-thirds by stirring every thirty minutes or oftener. The periods of motility are usually of short duration, while the quiet periods vary from five minutes to more than an hour. There is a tendency for some subjects to be less motile, hence more relaxed, during the early part of the night. Others may show a tendency to least motility in the middle part of the sleeping period or in the early morning. Nocturnal sleep following an afternoon nap may be deeper, though slightly delayed and shortened. The reason is probably not that suggested by Kohlschutter's curve, however, but the persistence of the habit of relaxation from the first sleep period into the second. The healthy child has a tendency to delay the onset of sleep longer than the adult and generally shows greater motility. In middle-aged adults the duration of quiet periods tends to increase with the length of time spent in bed. More important, however, is the tendency to spend a fairly constant time in quiet rest despite variation in the duration of sleeping period. The alternation of periods of motility and quiet is entirely individual and seems to fit no well-defined periodic law. Even when two individuals are placed in different beds under the same external conditions, their periods of motility and quiet do not agree more often than would be expected by chance. This last result indicates that external stimuli have relatively little effect upon sleep as compared with such conditions as the differential relaxation of the muscles, gastrointestional disturbances, residual excitation (as in so-called occupational delirium), and urinary and glandular changes.