Nature of Sleep

In the animal kingdom sleep is a common and essential biologic process to sustain and restore health. Sleep is defined as a physiologic and behavioral state characterized by partial isolation from the environment. A baby’s cry, the vibration of an earthquake, or sudden pain intrusion will all interrupt function to awaken the organism for protection purposes.

The length of sleep in human adults averages between 6 to 9 hours. Most adults sleep an average 7.5 hours where some are short sleepers and some are long sleepers ( less than 5.5 hours and more than 9.0 hours, respectively). Good sleep quality is typically associated with a sense of having slept continuously through the night and feeling alert and refreshed upon awakening in the morning. However, the perception of sleep quality is subjective and varies widely among people. Some individuals perceive their sleep as satisfying most of the time, and some routinely report being poor sleepers (feeling unrefreshed when they awaken, having difficulties in initiating or maintaining sleep, and having nightmares).

It is important for dentists entering the field of dental sleep medicine to identify sleep disorders, ( bruxism, snoring, obstructive sleep apnea, gastroesophageal reflux), and pain interference. An understanding of the nature of sleep is imperative to the dentist’s role in management of such problems.

The diagnosis and management of sleep disorders are interdisciplinary.  Dr. Nugent works with a team of medical doctors for optimal patient care.

Functions of Sleep


  • Daytime learning needs sleep for memory consolidation.
  • Sleep seems to facilitate programming of new information.

Fatigue reversal

  • Sleep allows the person to recover and reenergize.

Immune Function

  • Reset or protection.

Biochemical refreshment

  • Sleep promotes synaptic efficiency, protein synthesis, neurogenesis, metabolic (eg, glycogen) restoration, growth (secretion of growth hormone peaks during sleep) etc.

Psychologic well-being

  • Dreams occur in all sleep stages. REM dreams are more vivid.
  • Lack of sleep presents a risk of mood alteration to depression.

 Sleep and Health

Sleep brings about several functions, including physical recovery, memory consolidation, biochemical refreshment (synaptic function) and emotional regulation. Sleep deprivation is insufficient sleep resulting from short sleep duration or loss of a sleep segment because of a contributing medical condition (pain or diabetes) or environmental factors (noise).

An experiment with young individuals comparing the ramifications of sleep deprivation (4 hours of sleep over 3 to 4 days) to the effects of the subjects’ usual 8 hours of sleep revealed that sleep deprivation triggers sociability dysfunction, mood alteration, and complaints of bodily pain. For parents of kids this should be no surprise as I am sure most parents have witnessed a cranky kid after just one night of not getting the proper amount of sleep. In review, a persistent reduction in sleep duration can cause physical and mental health problems because of the cumulative effect of lack of sleep on several physiologic functions.

Furthermore, both too-short and too-long sleep durations have been associated with higher risks of diseases and mortality. The complicated connections among lifestyle, morality risk, and sleep duration are not fully understood. There is evidence to support the relationship between sleep duration (too little or too much) and the risk of cardiovascular disease (such as myocardial infraction and atherosclerosis), diabetes, obesity, depression, and even cancer. While these risks estimates are modest, they have been reproduced in too numerous studies to reject the reputed effect of cumulative sleep debt on health maintenance. Higher risks of myocardial infraction have been in women who are short sleepers as well as women who are long sleepers. Elevated risks of cardiovascular problems and atherosclerosis also have been reported in people who sleep too much during the day.

Because dentists are health professionals who see a large percentage of the population annually for dental checkups, dentists are in an excellent position to convey the message of the importance of good sleep habits for overall health and to screen for sleep disorders.

Sleep-Wake Cycle

An adult’s 24-hour cycle is divided into 16 hours of wakefulness and 8 hours of sleep. Synchronization and equilibrium between the sleep- wake cycle and feeding behaviors are crucial for survival. Mismatches in the synchronization of feeding cue and metabolic activity are associated with eating disorders. Poor sleep can cause health problems and can increase the risk of transportation (drowsy driving), work-related accidents and even death.

Homeostatic Process

The disposition to sleep is directly dependent on the duration of the prior wakefulness episode. As the duration of wakefulness increases, sleep pressure accumulates and builds to critical point, when sleep onset is reached. The ongoing 24-hour circadian rhythm therefore runs parallel to the homeostasis process, also known as process S. The S process corresponds to the sleep pressure that individuals accumulate during the wakefulness period before being able to fall asleep. With increasing sleep pressure, sleep is proportionally longer and deeper in the following recovery period.

Circadian Rhythm

Humans tend to alternate between a period of wakefulness lasting approximately 16 hours and a continuous block of 8 hours of sleep. Most mammals sleep around a 24-hour cycle that is driven by clock genes that control the circadian rhythm. Light helps humans synchronize their rhythm with the cycles of the sun and moon by sending the retinal signal (melanopsin) to the hypothalamic suprachiasmatic nucleus. The suprachiasmatic nucleus is a network of brain cells and genes that acts as a pacemaker to control the circadian timing function.

Ultradian Rhythm

Under the 24-hour process C of sleep and wakefulness, sleep onset and maintenance are governed by an ultradian cycle of three to five periods in which the brain, muscles, and autonomic cardiac and respiratory activities fluctuate. These cycles consist of rapid eye movement (REM) sleep (active stage) and non-REM sleep (light and deep stages)

In humans, a clear decline in electrical brain and muscle activities as well as heart rhythm is observed from wakefulness to sleep onset. This decline is associated with a synchronization of the brain waves towards stage 1 sleep. Stage 1 is a transitional period between wakefulness and sleep. Stage 2 sleep then begins, accounting for about 50% to 60% of total sleep duration. Stage 2 sleep is characterized by two EEG signals, K-complexes (brief, high amplitude brain waves) and spindles (rapid, springlike EEG waves), both of which are described as sleep-promoting and sleep-preventing factors. Sleep stage 1 and 2 are categorized as light sleep.

Next, sleep enters a quiet period known as deep sleep, or stage 3 and 4. These stages are characterized by slow, high-amplitude brain wave activities. Stages 3 and 4 are usually scored together and are characterized by a dominance of slow-wave activity (delta sleep = 0.5 to 4.5 Hz). This sleep period is associated with a so-called sleep recovery process.

Finally, sleep enters an ascension period and rapidly turns into either light sleep or REM sleep.  REM sleep is associated with reduction in the tone of postural muscles (which is poorly described as “atonia” in literature but is in fact hypotonia because muscle tone is never zero) and rise in heart rate and brain activity to levels that frequently surpass the rate observed during wakefulness. Humans can dream in all stages of sleep, but REM dreams may involve intensely vivid imagery with fantastic and creative content. During REM sleep.  The body is typically in a paralyzed-like state (muscle hypotinia). Otherwise, dreams with intense emotional content and motor activity might cause body movement that could injure individuals and their sleep partners.

A knowledge of the presence of ultradian sleep cycles is relevant because certain pathologic events occur during sleep, including the following sleep disorders:

  • Most periodic body movements (leg or arm) and jaw movements, such as sleep bruxism, are observed in stage 2 sleep and with less frequency in REM sleep
  • Sleep-related breathing events, such as apnea and hypopnea (cessation or reduction of breathing), are observed in stage 2 and REM sleep.
  • Acted dreams with risk of body injury, diagnosed as sleep movement disorder REM behavior disorder, occur during REM sleep

Sleep Recordings and Sleep Arousal

When a polygraphic sleep test of patient (collected via a HST or PST) is assessed, the scoring of sleep fragmentation is key element in assessing sleep quality. Poor sleep quality, as reported subjectively by the patient, is associated with frequent arousals, with or without body movements, frequent stage shifts (from a deeper to a lighter sleep stage), respiratory disturbances, and higher muscle tone. All these signs of sleep fragmentation interrupt the continuity of sleep and alter the sleep architecture.

Sleep efficiency is another main variable to evaluate through sleep tests. A standard index of sleep impairment, sleep efficiency id defined as the amount of time asleep divided by the amount of time spent in bed, expressed as a percentage. Sleep efficiency greater than 90% is an indicator of good sleep.

The ultradian cycle of sleep includes another repetitive activity: sleep-related arousals. During non-REM sleep, arousals are recurrent (6-14 times per hour of sleep), involving brief (3 to 10 seconds) awakenings associated with increased brain, muscle, and heart activities (tachycardia, or rapid heart rate) in the absence of the return of consciousness. In the presence of the sleep movements, breathing disorders, or chronic pain, these arousals are more frequent. Sleep arousals are thought to be the body’s attempt to prepare the sleeping individual (who is in a low-vigilance state) to react to a potential risk, ie, a fight-or-flight state.

Sleep arousals exist with or precede most periodic limb movements and sleep bruxism. In contrast, sleep apnea and hypopnea are respiratory distress-like events that trigger sleep arousals. An index of motor events, respiratory disturbances, and frequency of shifts in sleep stage can be calculated to assess the presence of periodic limb movements, bruxism, snoring, and sleep-related apnea and hypopnea.

People appear to have different levels of tolerance for sleep fragmentation. These levels may be genetically determined. Nevertheless, recurrent sleep deprivation or fragmentation produces a cumulative sleep debt. This sleep debt is likely to increase complaints of fatigue, memory and mood dysfunction, and bodily pain. The cause-and-effect relationship remains to be confirmed.

Most adults sleep between 6 to 7 hours on work days and more on weekends. By about the age of 40 years, adults’ sleep starts to become more fragile. Individuals are more aware of being awake for a few seconds to a few minutes a night. In the elderly, the sleep-wake pattern returns to a multiphase pattern typical of young children. Elderly people go to sleep earlier than middle-aged adults and awaken earlier in the morning taking occasional naps (catnapping) during the day.

Good-quality sleep allows humans a means of physical recovery, biochemical refreshments, memory consolidation, and emotional regulation. The diagnosis, prevention, and management of disorders that interfere with the quality of sleep are vital to the general health of the public. Dentists should work with other health professionals to improve sleep disorder management for their patients. Strategies to improve the efficacy of the sleep-wake process, such as light exposure, exercise, general sleep hygiene, a relaxing situation, and elimination of sleep disorders can all be done with a sleep trained dentist.

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