Sleep occurs because of the happy coincidence of two natural forces. These are the accumulation of a sleep debt or a need to sleep due to a period of prior wakefulness and a daily physiological urge to sleep that comes as a part of the body’s physiological 24-hour rhythm. Our sleep debt starts accumulating from the moment we stop sleeping and increases the longer we are awake. It is the price we pay for wake and sleeping pays it off. We usually need about 8 hours of sleep to fuel wakefulness for the next 16 hours. After 16 hours, it is sufficiently strong to support sleep onset. Without dissipation by sleep, our sleep debt will increase in strength but reaches a maximum level after about 24 hours and will dominate all of life thereafter.
Without sleep we would eventually die. The drive for sleep is so strong that sleep occurs long before death is a real consequence. We know that you can kill rats with severe sleep deprivation and there have been extremely rare cases of fatal insomnia in humans.
Rest from activity on a daily basis is a fairly universal trait among all animals. We differ from most of the simpler animals on earth by possessing the ability to regulate our body temperature and share with all other warm-blooded animals (mammals and birds) a natural state of unconsciousness on a daily basis that we call sleep. The cold-blooded creatures rest but the brain structures to support sleep are largely absent. There is no convincing evidence of sleep with its unique brain activity amongst reptiles and lesser animals.
The body organizes itself to accomplish specific tasks at set intervals within the 24-hour day. Darkness is the preferred time for sleep and rest. Our body prepares for sleep by orchestrating a complex set of physiological changes that favor wake activity in daytime and sleep at night in a rhythmic manner. This includes modulation of hormone and neurotransmitter levels (i.e. body temperature, blood pressure, heart rate, and cellular events of varied complexity). The net results is physiological support of the sleep process as our daytime activity ends. Sleep induction is also favored as our sleep debt reaches a significant level after about 16 hours of wakefulness. The merging of our sleep debt pressure with physiological support of sleep results in a strong sleep urge at bedtime.
WHAT IS NORMAL SLEEP?
Sleep is associated with quiescent behavior, a reduced responsiveness to stimulation, profound changes in brain electrical patterns and significant changes in physiology. Two dramatic changes associated with the transition from wake to sleep are amnesia for thought and a loss of volitional control of thought processes. Sleep also contains a peculiar phenomenon called dreaming where there is a strange admixture of real life experience accompanied by bizarre distortions of time, place and physical limitations. Dreams spontaneously occur within sleep about every 90 minutes throughout the night in a paralyzed body but with a very active mind that moves through scenes that are impossible to predict and often without much of a clue as to “Why this?”.
HOW IS NORMAL SLEEP ACHIEVED?
The experience of natural sleep requires that one is observing at least four fundamental habits of good sleep hygiene, which are:
- maintaining a regular evening bedtime.
- sleeping about 7.5 hours nightly and remaining awake by day.
- sleeping in a dark, quiet and comfortable location.
- abstinence from stimulating or sedating herbs and medicines.
Any exceptions to these assumptions will preclude normal sleep as we are defining it. Therefore, the onset of natural sleep is a predictable and pleasant experience, which requires a recumbent position and a favorable environment and lasts for about eight hours. Lastly, it is terminated spontaneously without aid and results in a refreshed body and mind.
HOW DID WE COME TO UNDERSTAND OUR CURRENT CONCEPTS ON SLEEP?
We knew very little scientifically about sleep before 1950 and little about the disorders of sleep before 1970. It was the discovery in 1953 of a second state of sleep uniquely associated with dream reports that marked the beginning of an explosion of new information about the science of sleep. We now know that during dreams there is a redirection of the brain’s neural firing patterns. These include occasional bursts of conjugate and rapid eye movements, skeletal muscle paralysis, brain wave patterns that look close to those of wakefulness and spontaneous irregularities in autonomic control of breathing and heart rate. It was the later discovery in the late 1960’s that significant breathing abnormalities in sleep accounted for severe pathological cardiovascular consequences and profound daytime sleepiness that clinical services for sleep problems developed. There followed the rise of sleep disorder centers, the development of a classification system for sleep disorders and the birth of a new discipline called sleep medicine. This assured further scientific study of the sleep process.
HOW IS THE TIMING OF SLEEP CONTROLLED?
The timing of sleep is predictable. It occurs about every 24 hours but changes in light-dark cycles and other environmental queues influence its timing. Bright light in the few hours before sleep onset tends to delay the time of sleep onset and also delay awakening the next morning. Thus, bright light in the bedroom at bedtime can phase delay sleep within the 24-hour day. On the other hand, bright light exposure at the end of the normal sleep period tends to move sleep the other direction (backwards or earlier) in the 24-hour period or advance the phase of sleep. Thus, third shift workers find their urge to sleep inhibited by light exposure at the end of their work shift. This same mechanism of light sensitivity is used by animals to determine their daily schedule. Essentially, they start their day at sunrise and end their day at sunset. Therefore, they can adapt to seasonal changes in day length or to time zone shifts during migration. We can use the same principles to adjust when we sleep if light exposure is systematically applied.
WHY DO WE SLEEP IN DARK, QUIET COMFORT?
In every brain there is a core of neurons in the lower brainstem that is necessary for vital functions such as breathing, sleeping, and wakefulness referred to as the reticular formation. Destruction of this tissue will result in a permanent comatose condition. This collection of neurons is referred to as the waking center of the brain. Stimulation of the waking center arouses from sleep and prevents sleep onset. Sleep requires that we minimize sensory stimulation because it drives the waking center of the brain and postpones the normal drive for sleep onset. For this reason, we seek a quiet, comfortable, and dark environment and a disengagement from energetic activity or stress.
Stimulation of the waking center of the brain can come from two general sources. One is intrinsic from our own thought processes. We do not readily fall asleep when we are worried, angry, tense or in mental turmoil or excitement for any reason. These kinds of thought processes are coupled with release of adrenaline via the sympathetic nervous system. Once released, adrenaline requires about 30 minutes to be metabolized to remove its alerting effect. This is why a time to relax between activity and sleep is suggested. For those who are having trouble controlling thoughts, meditation, pleasant reading, soft slow music and other relaxing techniques may be recommended. The other source of stimulation to the waking center comes from the numerous sensory pathways that ascend to the sensory cortex. These pathways also feed parallel projections to the waking center to improve the cortex’s ability to translate their input. A large part of the brain’s cortex is devoted to processing such sensory information. Pain receptors are widely distributed all over the body both at the surface and internally. We also are sensitive to touch, position through stretch receptors and temperature. There are several large cranial nerves devoted entirely to sensory reception including, cranial nerve I for smell, II for vision, most of V for sensation to the face and head, VIII for hearing and balance, and a major portion of X for gut sensations. Sleep is favored when the sensory cortex and the waking center is not fed information. Closing the eyes rests the visual cortex, a comfortable temperature minimizes thermal distress, a relaxed position puts position receptors in a neutral mode and we minimize sensory input. Our internal thought processes, the other threat, need to be peaceful to disengage the sympathetic nervous system and its alerting power.
WHAT HAPPENS AT SLEEP ONSET?
When preceded by the above precautions, sleep onset is a spontaneous and pleasant experience. It should not require sedation to achieve. When we lay down to sleep, brain wave patterns (EEG or electroencephalogram) of wake reflect a firing of neurons in rapid but asynchronous manner referred to as a low voltage fast EEG tracing. When we close our eyes, the eye muscles producing coordinated and focused vision relax and the eyes roll into a relaxed position. Brain wave patterns over the visual cortex at the back of the head develop a high degree of synchronous activity called alpha waves. Usually within 10 minutes there follows an increase in synchronous firing of neurons over the entire brain surface with theta waves (3-7 cps) replacing the waking brain pattern over the entire cortical surface and sleep onset is judged to have occurred. If awakened at this moment, most subjects will report that they are not asleep but in relaxed wakefulness. It requires about 10 minutes of sleep to lose direction of the thought process and amnesia for wake. Conscious thought does not stop at sleep onset. Therefore, it is common to believe that you are awake at sleep onset and to believe you have been awake all night if sleep is highly fragmented by frequent wake episodes.
Loss of consciousness takes several minutes to occur after sleep onset and will reduce your responsiveness to all other senses including hearing. This is why falling asleep in front of the television can occur even though we think we are in touch with the program. We first lose visual contact and then the loss of audio contact will occur several minutes later when we progress deeper into sleep. Remember this the next time you are driving while drowsy. You can have lots of denial and you can fall asleep and believe you are still awake and driving for a few minutes. If you do not arouse, sleep will progress and with disastrous consequences. In summary, the process of transitioning from wake to unconscious sleep may require about 20 minutes on a typical night.
WHAT HAPPENS AFTER SLEEP ONSET?
Sleep is not homogeneous. Based upon assignment of brain wave patterns into 5 stages, Figure 1, sleep is seen to have a pattern that repeats in 90-minute units across the night. During the first hour of sleep, we move through successive stages of non-dreaming sleep. It begins with stage 1 at sleep onset. In it there are theta rhythms (3-7 cps) in the cortical tracings. Often within about 10 minutes a new phenomenon appears called sleep spindles because they resemble a spindle of thread. They are tightly packed bursts of faster activity (12-14 cps) and occur about every 10 to 20 seconds against the background of theta activity. Another unusual activity pattern emerges in parallel with the sleep spindle and is called a k-complex. It is of high amplitude and typically shows a sharp rise in negative polarity followed by a positive swing in the opposite direction. They can occur spontaneously either once or several times per minute. They can also be induced by noise and are thought to reflect activity in the auditory pathway. The presence of a sleep spindle or a k-complex signals the beginning of stage 2 sleep. Some believe that their appearance marks the approximate time when loss of conscious direction of thought and amnesia for wakefulness occurs. As sleep progresses, the background brain wave pattern usually slows to about 2 cps and gains in amplitude. When a threshold of at least 20% of the tracing reaches a slow high amplitude pattern, stage 3 sleep is scored and at 50% of the tracing, stage 4 sleep is obtained. As the 90-minute mark is approached, there is a reversal of this pattern and stage 4 yields to stage 3 and then back to stage 2 followed by a transition to dreaming or REM sleep.
REM (rapid eye movement) sleep is defined by the simultaneous convergence of three rather dramatic changes. These are the loss of sleep spindles, k-complexes and slow waves with a pattern close to stage 1 in the EEG, the complete loss of skeletal muscle tone and episodic bursts of conjugate and rapid eye movements. The first dream episode is rather brief and may last only about 10 minutes. It is followed by another cycle of sleep like the first and transitions into another REM sleep episode, which will be a little longer. Thereafter, the interval between dreams is usually limited to only stage 1 and 2 sleep and the dreams themselves continue o expand in length. A typical sleep period will contain four dream episodes that constitute about 25% of the night’s sleep. Stage 1 is usually limited to about 5% of sleep time, stage 2 is 50% of sleep and stage 3 and 4 combine to make up about 20% of sleep. These would represent quotas for a healthy young adult.
HOW DOES SLEEP END?
Sleep usually terminates at the end of the last dream episode. There is a spontaneous transition to wakefulness associated with the feeling that you are through sleeping. Your body temperature, heart rate and blood pressure have been steadily declining toward their daily nadir while you slept. There will be a small but definite rise in heart rate and blood pressure when you stand up and a rise in body temperature. Your baseline heart rate, blood pressure and body temperature will each steadily rise to their peak values by late afternoon along with most mental and motor function skills. It is these inherent changes associated with other physiological processes that support wakefulness. It is why daytime sleep is usually terminated prematurely in third shift workers who try to sleep against this underlying physiological urge to wake up.
This is the gist of normal sleep. You may have lots of questions. Please feel free to ask or read other headings that may answer your questions.