Editing Sleep (section)

=== Circadian clock ===
{{Main|Circadian rhythm}}
{{further|Circadian rhythm sleep disorder}}
[[image:Biological clock human.svg|402px|thumb|The human "[[Circadian rhythm|biological clock]]"]]

Sleep timing depends greatly on [[hormone|hormonal]] signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in [[diurnality|diurnal]] animals) and augments it at night.<ref name=FullerEtAl2006>{{cite journal | vauthors = Fuller PM, Gooley JJ, Saper CB | title = Neurobiology of the sleep-wake cycle: sleep architecture, circadian regulation, and regulatory feedback | journal = Journal of Biological Rhythms | volume = 21 | issue = 6 | pages = 482–93 | date = December 2006 | pmid = 17107938 | doi = 10.1177/0748730406294627 | s2cid = 36572447 }}</ref><ref name=WaterhouseEtAl2012 /> The [[suprachiasmatic nucleus]] (SCN), a brain area directly above the [[optic chiasm]], is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body.

An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be ''[[Entrainment (chronobiology)|entrained]]''; an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions as [[Free-running sleep|free-running]] of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day.<ref name=RonnenbergEtAl2007 /><ref>{{cite book | author-link1 = Phyllis C. Zee | vauthors = Zee PC, Turek FW | date = 1999 | chapter = Introduction to Sleep and Circadian Rhythms | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 1–17 }}</ref><ref name=DijkEdgar1999>{{cite book | vauthors = Derk-Jan D, Edgar DM | date = 1999 | chapter = Circadian and Homeostatic Control of Wakefulness and Sleep | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 111–147 }}</ref>

The circadian clock exerts constant influence on the body, affecting [[Sine wave|sinusoidal]] oscillation of [[thermoregulation|body temperature]] between roughly 36.2&nbsp;°C and 37.2&nbsp;°C.<ref name=DijkEdgar1999 /><ref name=CzeislerWright1999>{{cite book | vauthors = Czeisler CA, Wright Jr KP | date = 1999 | chapter = Influence of Light on Circadian Rhythmicity in Humans | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 149–180 }}</ref> The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night.<ref name=ZlomanczukSchwartz1999>{{cite book | vauthors = Zlomanczuk P, Schwartz WJ | date = 1999 | chapter = Cellular and Molecular Mechanisms of Circadian Rhythms in Mammals | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 309–342 }}</ref> The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the [[pineal gland]], which releases the hormone [[melatonin]] at night.<ref name=ZlomanczukSchwartz1999 /> [[Cortisol]] levels typically rise throughout the night, [[Cortisol awakening response|peak in the awakening hours]], and diminish during the day.<ref name="VanCauterSpiegel1999"/><ref name=Wehr1999>{{cite book | vauthors = Wehr TA | date = 1999 | chapter = The Impact of Changes in Nightlength (Scotoperiod) on Human Sleep | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 263–285 }}</ref> Circadian [[prolactin]] secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone.<ref name=VanCauterSpiegel1999 />

The circadian rhythm influences the ideal timing of a restorative sleep episode.<ref name=RonnenbergEtAl2007 /><ref name=Wyatt1999>{{cite journal | vauthors = Wyatt JK, Ritz-De Cecco A, Czeisler CA, Dijk DJ | title = Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day | journal = The American Journal of Physiology | volume = 277 | issue = 4 Pt 2 | pages = R1152-63 | date = October 1999 | pmid = 10516257 | doi = 10.1152/ajpregu.1999.277.4.r1152 | s2cid = 4474347 | quote = ... significant homeostatic and circadian modulation of sleep structure, with the highest sleep efficiency occurring in sleep episodes bracketing the melatonin maximum and core body temperature minimum <!-- the quote is only relevant to one time that this ref is used --> }}</ref> Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas [[slow-wave sleep]] can occur more independently of circadian time.<ref name=DijkEdgar1999 />

The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at [[phase response curve|the right moment]] in the circadian cycle, can significantly 'reset' the internal clock.<ref name=CzeislerWright1999 /> Blue light, in particular, exerts the strongest effect,<ref name=WaterhouseEtAl2012 /> leading to concerns that [[Electronic media and sleep|use of a screen]] before bed may interfere with sleep.<ref name="ChangAM">{{cite journal | vauthors = Chang AM, Aeschbach D, Duffy JF, Czeisler CA | title = Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 4 | pages = 1232–7 | date = January 2015 | pmid = 25535358 | pmc = 4313820 | doi = 10.1073/pnas.1418490112 | bibcode = 2015PNAS..112.1232C | doi-access = free }}</ref>

Modern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially [[Shift work|night shifts]]), long-distance travel, and the influence of universal indoor lighting.<ref name=DijkEdgar1999 /> Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle.<ref name=AkerstedtEtAl2002 /> A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise.<ref name=DijkEdgar1999 />