THE SLEEP STAGES

THE SLEEP STAGES Initial discoveries

The next step in the evolution of understanding sleep was the recognition that EEG brainwaves, when combined with other kinds of physiological information, could be used to identify rhythmically recurring, discrete sleep stages. The first description of these sleep stages was made by the American scientist Alfred Loomis (1887–1975). 

As a young man in the army he developed the Aberdeen Chronograph, a system for measuring projectile velocity by firing a bullet through revolving paper-covered aluminum disks. He went on to a successful career in investment banking. 

Becoming restless once again, and still remembering his success with the chronograph, he turned his attention to developing radar for military purposes and for ground control during the landing approach of aircraft. He was fascinated with the measuring of waveforms and, among the many projects at his laboratory at Tuxedo Park, New York, was the study of sleep. Using a large 8 foot (2.4 m) diameter recording drum, he described in 1937 a series of five discrete recurring sleep stages during the night, which he rather unpoetically designated as stages A–E. 

Rapid Eye Movements: An example of EOG (electrooculogram) channels in a polysomnogram showing the rapid eye conjugate eye movements that occur during REM sleep. The discovery of REM sleep represented an exciting breakthrough in sleep research

In terms of later development, stages A and B correspond roughly to what was later called stage 1, C corresponded to stage 2, and D and E resembled slow-wave sleep. All together they correspond to what we now call non-rapid eye movement (NREM) sleep. The discovery of rapid eye movement (REM) sleep The next big development, which in effect ushered in the modern age of sleep research, occurred in the early 1950s. 

Nathaniel Kleitman (1895–1999), a physiologist at the University of Chicago, had been interested in eye movements and blinking as a marker of sleep onset and depth of sleep, as well as possible rhythmic behaviors, in infants. He enlisted the aid of a graduate student, Eugene Aserinsky (1921–98). Following observations in infants, they adapted the technique of the electrooculogram (EOG) for continuous use in sleep of children and adults. 

In the process of doing so, they observed the periodic appearance of vigorous and jerky ocular activity. This new stage, known as rapid eye movement (REM) sleep, was characterized not only by the eponymous eye movements, but also by relaxation of the major weight-bearing muscles, irregularity of respiratory and heart rate, and loss of temperature control. 

It also has psychological counterparts, and most dreaming, in the conventional sense of the word, occurs in REM. Indeed, REM sleep is as different from the rest of sleep (dubbed by Kleitman as non-REM or NREM sleep) as NREM is from waking. 

This has led some authors to describe humans as having three distinct states of consciousness: waking, REM, and NREM sleep. The sleep stages do not appear randomly, but instead are manifest in a rhythmic, repetitive pattern throughout the night. There are a number of influences on the appearance and duration of the individual stages, including a basic approximately 90–100-minute innate rhythmic cycle of NREM and REM sleep (an example of “ultradian rhythms,” see page 75), the time of the 24-hour day at which sleep occurs, and the duration of wakefulness before sleep. 

In the next section we will describe the sleep stages in much more detail, but perhaps the important message at this point is that sleep is not a unitary process, but is comprised of two very distinct states as well as several distinct stages in NREM sleep.