Editing Brain (section)

=== Sleep ===
{{Main|Sleep}}
{{See also|Circadian rhythm|arousal}}

Many animals alternate between sleeping and waking in a daily cycle. Arousal and alertness are also modulated on a finer time scale by a network of brain areas.<ref name="Kandel 2000" /><!--Ch. 45--> A key component of the sleep system is the [[suprachiasmatic nucleus]] (SCN), a tiny part of the hypothalamus located directly above the point at which the [[optic nerves]] from the two eyes cross. The SCN contains the body's central biological clock. Neurons there show activity levels that rise and fall with a period of about 24 hours, [[circadian rhythm]]s: these activity fluctuations are driven by rhythmic changes in expression of a set of "clock genes". The SCN continues to keep time even if it is excised from the brain and placed in a dish of warm nutrient solution, but it ordinarily receives input from the optic nerves, through the [[retinohypothalamic tract]] (RHT), that allows daily light-dark cycles to calibrate the clock.<ref>{{cite journal |last1=Antle |first1=MC |title=Orchestrating time: arrangements of the brain circadian clock |journal=Trends in Neurosciences |year=2005 |volume=28 |pages=145–151 |url=http://www.columbia.edu/cu/psychology/silver/publications2/149%20antle%20et%20al.pdf |pmid=15749168 |doi=10.1016/j.tins.2005.01.003 |last2=Silver |first2=R |issue=3 |s2cid=10618277 |url-status=dead |archive-url=https://web.archive.org/web/20081031120051/http://www.columbia.edu/cu/psychology/silver/publications2/149%20antle%20et%20al.pdf |archive-date=2008-10-31 }}</ref>

The SCN projects to a set of areas in the hypothalamus, brainstem, and midbrain that are involved in implementing sleep-wake cycles. An important component of the system is the [[reticular formation]], a group of neuron-clusters scattered diffusely through the core of the lower brain. Reticular neurons send signals to the thalamus, which in turn sends activity-level-controlling signals to every part of the cortex. Damage to the reticular formation can produce a permanent state of coma.<ref name="Kandel 2000"/><!--Ch. 45-->

Sleep involves great changes in brain activity.<ref name="Kandel 2000"/><!--Ch. 47--> Until the 1950s it was generally believed that the brain essentially shuts off during sleep,<ref>{{cite book|last=Kleitman|first=Nathaniel|title=Sleep and Wakefulness|publisher=The University of Chicago Press, Midway Reprint|others=Revised and enlarged edition 1963, Reprint edition 1987|orig-date=1939|date=1987|isbn=978-0-226-44073-6|location=Chicago}}</ref> but this is now known to be far from true; activity continues, but patterns become very different. There are two types of sleep: ''[[Rapid eye movement sleep|REM sleep]]'' (with [[dream]]ing) and ''[[Non-rapid eye movement sleep|NREM]]'' (non-REM, usually without dreaming) sleep, which repeat in slightly varying patterns throughout a sleep episode. Three broad types of distinct brain activity patterns can be measured: REM, light NREM and deep NREM. During deep NREM sleep, also called [[slow wave sleep]], activity in the cortex takes the form of large synchronized waves, whereas in the waking state it is noisy and desynchronized. Levels of the neurotransmitters [[norepinephrine]] and [[serotonin]] drop during slow wave sleep, and fall almost to zero during REM sleep; levels of [[acetylcholine]] show the reverse pattern.<ref name="Kandel 2000"/><!--Ch. 47-->