Elsevier

Critical Care Clinics

Volume 24, Issue 3, July 2008, Pages 449-460
Critical Care Clinics

Normal Sleep and Circadian Processes

https://doi.org/10.1016/j.ccc.2008.02.002Get rights and content

The onset of sleep is associated with a variety of changes in both behavioral and physiologic states. Sleep is not a uniform state either: it has different stages that affect different areas of the brain and body. Nonrapid eye movement sleep stages are as different from rapid eye movement sleep as is wakefulness. Circadian rhythms of physiologic systems also impact wake, sleep, sleepiness, and alertness. There are characteristic changes in both sleep patterns and circadian rhythm that occur with aging. The cardiovascular, respiratory, endocrine and gastrointestinal systems also undergo changes with sleep onset. This article reviews the aspects of normal sleep, physiologic changes that occur in the human body with sleep, and how sleep changes over the lifespan.

Section snippets

Normal sleep staging

Sleep is an essential physiologic process for most living organisms. Sleep is divided into nonrapid eye movement (NREM) sleep and rapid eye movement sleep (REM). Older sleep staging developed by Rechtschaffen and Kales [1] divided NREM sleep into four stages, numbered 1, 2, 3, and 4. According to the newly released American Academy of Sleep Medicine scoring criteria, NREM sleep is now characterized by three stages (N1, N2, N3), with N3 encompassing the older classification of stages 3 and 4 [2]

Sleep-wake cycles: a delicate symphony

The sleep-wake state is a delicate interplay of neurobiologic systems (Table 1). The Borbely two-process model is the most accepted theory, which describes the orchestrated balance necessary to achieve a normal sleep-wake cycle. The two-process model consists of the sleep-wake propensity, resulting from a combination of an intrinsic circadian pacemaker (process C) and a homeostatic process (process S). An individual's homeostatic sleep drive is directly determined by the duration of

Neurophysiologic centers essential to sleep-wake cycles: overivew

The sleep and wake state are both active neurophysiologic states involving several neurotransmitters that project to numerous areas along the neuroaxis (see Table 1). The posterior hypothalamus, tuberomammillary nucleus, and regions of the brain stem are all collectively involved in maintaining wakefulness with the release of excitatory neurotransmitters orexin, histamine, and acetycholine, respectively, to specific cortical and subcortical sites. During wakefulness, adenosine, a normal

Sleep-wake cycles over the lifespan

From birth to age 1 the sleep stages are less clearly defined. Infants typically go to sleep via “active sleep,” which is most similar to adult REM sleep. Sleep periodicity is less, typically 50 to 60 minutes, and is throughout the 24-hour period rather than consolidated into nighttime. As infants become young children, they develop more slow wave sleep, which may comprise one third to one half of the sleep period. Slow wave sleep decreases dramatically during the second decade and that

Cardiovascular

NREM and REM states profoundly affect cardiovascular processes. In NREM sleep, there is autonomic stability with dominance of parasympathetic tone and vagal nerve input. This results in prominent sinus arrhythmia, with coupling of respiratory variation. During inspiration, heart rate increases to accommodate the increased venous return and during expiration, heart rate slows. This pattern is normal and suggests normal functioning; reduced heart rate variability is seen in disease states and

Summary

Sleep is associated with distinct changes in not only the central nervous system but all physiologic systems. These distinct changes vary considerably between the different stages of sleep and become altered more with aging. In subsequent articles in this issue, it will be demonstrated how sleep deprivation, medications and different sleep disorders disrupt the delicate balance of these physiologic systems.

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