HUMAN SLEEP

HUMAN SLEEP

W e can measure our lives on many different levels. A life, for instance, could be considered the period in which a person’s heart beats a certain number of times (perhaps 3 billion), or enjoys Saturday nights (about 3,600), or experiences periods of peacefulness (all too few), or feels the joy of love (hopefully at least once). In the same way, sleep, which occupies perhaps one-third of our existence, can be viewed on a number of different levels. 

It is a process that can be measured physiologically while at the same time understood as a psychological experience, and even a social behavior. In this book we will explore some of these aspects of sleep. Human sleep is a reversible period of decreased consciousness and responsiveness, comprised of two distinct states known as rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. 

Although the modern era of sleep research began in the 1950s with the description of REM sleep, its roots go back to the 1920s with the discovery of the human electroencephalogram, or EEG. Our growing understanding of sleep has been influenced by some remarkable individuals who (sometimes while looking for something else) made crucial observations about sleep, by developments in psychology and technology, and even by world events.

WHAT IS SLEEP? 

The many qualities of sleep

S leep is a period of recurring behavioral quiescence, which has several qualities, including decreased awareness of and responsiveness to the environment, diminished consciousness, and the rhythmic appearance of certain physiologic patterns (stages). 

It tends to occur at particular parts of the 24-hour day–night cycle, and at customary locations, both depending on the particular species and environment. It is reversible, distinguishing it from coma or ongoing anesthesia. It is also self-regulating: if one is deprived of sleep, there will be a drive to have increased “recovery sleep” to make up for the loss. 

It is necessary for life, and is present in all mammals. These qualities will be discussed later in the chapter. During sleep one becomes less aware of the surroundings; this is one of the qualities that distinguishes sleep from quiet wakefulness. An extreme case would be someone who falls asleep at the wheel, consequently running a red light. On the other hand, this process is not absolute; it is clear that we can process, and act on, sensory information during sleep. 

A new parent, for instance, can sleep through the noise of a truck driving by the house, but awaken quickly at the sound of the newborn baby crying. Similarly, in a laboratory situation, the volume of a sound required to wake a person up (the “auditory arousal threshold”) is much higher for a meaningless sound such as an electronic tone compared to a meaningful stimulus such as a phone ringing or hearing one’s name called. 

A sleeper, then, is able to process information about an incoming sound, and determine whether it is important or not. Again, the arousal response is dependent on a variety of factors including the sleep stage, the duration of wakefulness before sleep, and how far into sleep one is when the sound occurs. 

Some individuals are more likely to be awakened by low volume noises, and hence are considered to be “light sleepers.” Interestingly, people with insomnia often have a normal auditory arousal threshold, suggesting that insomnia is different from just light sleep. Some sleeping pills increase the auditory arousal threshold, raising the possibility that the medicated sleeper will not be aroused by, for instance, a smoke alarm. 

Time-lapse videos show that two sleepers adjust their positions in response to the other’s movements (whether the other sleeper is a pet or another human). In one fi lm, for instance, a man sleeping with a cat on the bed may turn on his side, with the cat then responding by settling comfortably in the warm nook behind his knees. In co-sleeping humans, an elbow in the ribs may result in an adjustment of the second person’s position. Thus, although sensory input is diminished during sleep, this is not absolute, and indeed during sleep we are able to take in and act on information to some degree.

DIMINISHED CONSCIOUSNESS The nature of diminished consciousness in sleep is by far the hardest to express, since consciousness itself is so poorly understood. 

When we speak of consciousness, we refer to our experience of self and the world. Perhaps even more fundamentally, consciousness can be defi ned as “our mode of access.” We can additionally speak of “acts” of consciousness, such as perceiving, willing, imagining, and the “contents” of these acts, such as perceiving a sunset, willing a meeting, or imagining a good outcome. 

Other eff orts to characterize consciousness are helpful. For example, consciousness has been described as a paradoxical state in which a person is simultaneously a subject who can experience things and an object perceived by oneself. Generally most of us recognize this when we speak of ourselves in both ways simultaneously, for instance when we say, “Sometimes I have to remind myself that...” or “I owe it to myself to... ”. 

In such phrases we recognize that we are both a being having experiences and at the same time we can picture ourselves as objects. Additional qualities of consciousness include consciousness as subjective and private (not available to another person); unitary (experienced by a single person); and characterized by a subjective “feel” of each experience, a “what it is like” aspect. 

The American philosopher Thomas Nagel (1937– ) illustrated the “feel” which is essential to consciousness in the following manner: although we may study and understand the neurophysiology of a bat, we can never know how the world is experienced by a bat. 

Some authors argue that a subjective experience such as consciousness cannot profi tably be studied by traditional scientifi c techniques; others believe that the fact that a phenomenon is experienced subjectively does not mean that it cannot be explored objectively. 

New developments in technology, including the use of brainimaging studies, are beginning to advance our understanding of the physiology behind consciousness, and researchers have developed a number of models of how it may occur. 

In addition to looking at normal sleep, neuroimaging studies of people who have been given hallucinogens such as psilocybin have contributed new theories of consciousness based on the notion of entropy—the degree of order and disorder in neural connections.