Editing Circadian rhythm (section)

=== Humans ===
[[File:Circadian rhythm.svg|thumb|upright=1.35|When eyes receive light from the sun, the pineal gland's production of melatonin is inhibited, and the hormones produced keep the human awake. When the eyes do not receive light, melatonin is produced in the pineal gland and the human becomes tired.]]
{{see also|Sleep#Circadian clock|Phase response curve#Light}}

Early research into circadian rhythms suggested that most people preferred a day closer to 25 hours when isolated from external stimuli like daylight and timekeeping. However, this research was faulty because it failed to shield the participants from artificial light. Although subjects were shielded from time cues (like clocks) and daylight, the researchers were not aware of the phase-delaying effects of indoor electric lights.<ref>{{MEDRS|date=November 2013}} {{cite journal | vauthors = Duffy JF, Wright KP | s2cid = 20140030 | title = Entrainment of the human circadian system by light | journal = Journal of Biological Rhythms | volume = 20 | issue = 4 | pages = 326–38 | date = August 2005 | pmid = 16077152 | doi = 10.1177/0748730405277983 }}</ref>{{Dubious|reason=This is part review, part study, needs to be checked for proper use of secondary source|date=July 2014}} The subjects were allowed to turn on light when they were awake and to turn it off when they wanted to sleep. Electric light in the evening delayed their circadian phase.<ref>{{cite journal | vauthors = Khalsa SB, Jewett ME, Cajochen C, Czeisler CA | title = A phase response curve to single bright light pulses in human subjects | journal = The Journal of Physiology | volume = 549 | issue = Pt 3 | pages = 945–52 | date = June 2003 | pmid = 12717008 | pmc = 2342968 | doi = 10.1113/jphysiol.2003.040477 }}</ref> A more stringent study conducted in 1999 by [[Harvard University]] estimated the natural human rhythm to be closer to 24 hours and 11 minutes: much closer to the [[Solar time|solar day]].<ref>{{cite journal|url=http://news.harvard.edu/gazette/1999/07.15/bioclock24.html| vauthors = Cromie W |title=Human Biological Clock Set Back an Hour|journal=Harvard Gazette|date=1999-07-15|access-date=2015-07-04}}</ref> Consistent with this research was a more recent study from 2010, which also identified sex differences, with the circadian period for women being slightly shorter (24.09 hours) than for men (24.19 hours).<ref name=":0">{{cite journal | vauthors = Duffy JF, Cain SW, Chang AM, Phillips AJ, Münch MY, Gronfier C, Wyatt JK, Dijk DJ, Wright KP, Czeisler CA | title = Sex difference in the near-24-hour intrinsic period of the human circadian timing system | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = Supplement_3 | pages = 15602–8 | date = September 2011 | pmid = 21536890 | pmc = 3176605 | doi = 10.1073/pnas.1010666108 | bibcode = 2011PNAS..10815602D | doi-access = free }}</ref> In this study, women tended to wake up earlier than men and exhibit a greater preference for morning activities than men, although the underlying biological mechanisms for these differences are unknown.<ref name=":0" />