Like most animals, humans' sleep and wakefulness are modeled by inner circadian rhythms and external cues, so-called zeitgebers (see also the article on chronotherapeutics on page 10), to a period of roughly 24 hours. In particular, sunlight is able to reset our inner clocks, thereby enabling us to adjust our daily rhythm, e.g. after intercontinental flights. Not sleep itself, but rather when we sleep is regulated by the suprachiasmic nucleus in the anterior hypothalamus, which functions like an internal clock and is affected by environmental inputs (again, especially sunlight, which reaches the hypothalamus via the retinohypothalamic tract) [1].
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| sleep well; source: http://bit.ly/2yFBDBZ |
Is sleep just a consequence of the brain being less active because it's tired? Although this explanation might appeal to many fellow PhD students, sleep is characterised by a complex pattern of brain activity! Key aspects of sleep are: little motor activity, little response to stimulation, typical postures (like lying down curled up) and the state being quite easily reversible (distinguishing sleep from coma, for example) [1]. These features can be monitored by conventional electrical recordings, including electromyography and electroencephalography (EEG). When people (and animals) fall asleep, EEG recordings show a drastic change in neuronal activity.
Sleep = Brain Activity?!
Broadly speaking, sleep comes in two flavors: REM (rapid eye movement) sleep and non-REM sleep, which consists of four stages of characteristic brain activity patterns. Wakefulness typically comprises approximately 20 Hz waves. When people fall asleep, this frequency falls to 10 Hz and entering sleep stage 1 is characterized by mixed frequency patterns, light muscle activity and slow rolling eye movements. Likewise, body temperature and metabolism slow down. The next stage (stage 2) is characterized by 12-14Hz activity sleep spindles and K complexes, biphasic high-voltage waves. The sleep stages 3 and 4 are also referred to as slow-wave sleep, as EEG recordings of these phases are dominated by delta waves of only 0.5-2Hz frequencies. In contrast to the characteristics of the four stages of non-REM sleep, brain activity in REM sleep resembles wakefulness, with some populations of neurons being even more active when you are in REM sleep than when you are awake. REM sleep is accompanied by an increase of body temperature and metabolic rate but an almost complete loss of muscle tone, except for the eyes which characteristically move rapidly [1].
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| Sleep phases; source: http://bit.ly/2zp9M61 |
Interestingly, it is easier to wake a person up during REM sleep than stage 3-4 of non-REM [2]. As brain activity during REM sleep pretty much resembles EEG patterns during wakefulness, it may not seem surprising that most dreaming occurs during these phases of a night’s sleep. Dreaming can also occur during non-REM sleep, although with a much lower incidence and slightly different characteristics [2].
Sleep is not a uniform State!
Each sleeper moves through REM and the 4 stages of non-REM sleep several times a night in cycles of 90-110 minutes. During a night’s sleep, the stages do not succeed eachother in a particular order and also change in length. For example, phases of REM sleep can take 1-60 minutes and may be accompanied by brief periods of waking [3]. The cycles through the different phases of sleep, the sleep “architecture”, differ between subjects, single nights, and also change with age. In early childhood, much more time sleeping is speny in deeper sleep stages 3 and 4 whereas in older age stage 2 sleep dominates [4]. (See also our article in "The Aging Brain")
How you sleep changes with age
But what's the point of such a complicated sleep architecture? As also discussed in the articles “Evolutionary basis of sleep” on page 5 and “Sleep and learning ” on page 6 of this issue, sleep serves important functions for the body and thus is necessary. However, being unresponsive to potential threats is a significant problem, which is in part circumvented by the fact that we alternate between periods of deeper and lighter sleep [3].
As you can see sleep is much more than just the most unproductive period between two days. It is quite complex and interesting and science still needs a lot of effort to unravel all its mysteries. Therefore, everyone should spend more time in personal field studies. For example, at home - sleeping.
Good Night!
[1] Kandel, Schwarz, Jessel. Principles of Neural Science, 2003
[2] Staunton, Naturwissenschaften, 2005
[3] Voss, Rev Neurosci. 2004
[4] Zepelin et al., J Gerontol, 1983
by Bettina Schmerl, PhD Student AG Shoichet
This article originally appeared Dedcember 2017 in CNS Volume 10, Issue 04, Sleep
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