Normal Sleep and Circadian Processes
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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