title: Entrainment of the human circadian clock to the natural light-dark cycle.
abstract: The electric light is one of the most important human inventions. Sleep and other daily rhythms in physiology and behavior, however, evolved in the natural light-dark cycle [1], and electrical lighting is thought to have disrupted these rhythms. Yet how much the age of electrical lighting has altered the human circadian clock is unknown. Here we show that electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14 hr 40 min:9 hr 20 min light-dark cycle camping. Furthermore, we find that after exposure to only natural light, the internal circadian clock synchronizes to solar time such that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise. In addition, we find that later chronotypes show larger circadian advances when exposed to only natural light, making the timing of their internal clocks in relation to the light-dark cycle more similar to earlier chronotypes. These findings have important implications for understanding how modern light exposure patterns contribute to late sleep schedules and may disrupt sleep and circadian clocks.
authors.affiliation: Sleep and Chronobiology Laboratory, Department of Integrative Physiology, University of Colorado Boulder, Boulder, CO 80309-0354, USA. kenneth.wright@colorado.edu
authors.forename: Kenneth P
authors.initials: KP
authors.lastname: Wright
authors.forename: Andrew W
authors.initials: AW
authors.lastname: McHill
authors.forename: Brian R
authors.initials: BR
authors.lastname: Birks
authors.forename: Brandon R
authors.initials: BR
authors.lastname: Griffin
authors.forename: Thomas
authors.initials: T
authors.lastname: Rusterholz
authors.forename: Evan D
authors.initials: ED
authors.lastname: Chinoy
mesh_terms: D000328:Adult; D057906:Circadian Clocks / Q000528:radiation effects*; D005260:Female; D006801:Humans; D008029:Lighting; D008297:Male; D017440:Photoperiod; D013472:Sunlight; D055815:Young Adult
references.citation: Front Mol Neurosci. 2012 Apr 18;5:50
references.pmid: 22529774
references.citation: Chronobiol Int. 2009 Jul;26(5):854-66
references.pmid: 19637047
references.citation: Sci Am. 1975 Jul;233(1):69-77
references.pmid: 1145170
references.citation: Neuro Endocrinol Lett. 2010;31(1):92-6
references.pmid: 20150866
references.citation: Photochem Photobiol. 1997 Nov;66(5):549-61
references.pmid: 9383985
references.citation: Annu Rev Physiol. 1993;55:16-54
references.pmid: 8466172
references.citation: Sci Transl Med. 2010 May 12;2(31):31ra33
references.pmid: 20463367
references.citation: J Physiol. 2001 Sep 15;535(Pt 3):937-51
references.pmid: 11559786
references.citation: J Biol Rhythms. 2002 Apr;17(2):181-93
references.pmid: 12002165
references.citation: Am J Psychiatry. 1985 Feb;142(2):163-70
references.pmid: 3882000
references.citation: Science. 1986 Aug 8;233(4764):667-71
references.pmid: 3726555
references.citation: Sleep Med. 2011 Aug;12(7):685-92
references.pmid: 21704557
references.citation: Curr Biol. 2007 Jan 23;17(2):R44-5
references.pmid: 17240323
references.citation: J Biol Rhythms. 2003 Jun;18(3):183-94
references.pmid: 12828276
references.citation: Curr Biol. 2012 May 22;22(10):939-43
references.pmid: 22578422
references.citation: J Biol Rhythms. 2010 Oct;25(5):329-39
references.pmid: 20876813
references.citation: Neurosci Lett. 2006 Mar 13;395(3):191-5
references.pmid: 16309837
references.citation: J Clin Endocrinol Metab. 1991 Dec;73(6):1276-80
references.pmid: 1955509
references.citation: J Physiol. 2000 Aug 1;526 Pt 3:695-702
references.pmid: 10922269
references.citation: Proc Natl Acad Sci U S A. 2013 Apr 2;110(14):5695-700
references.pmid: 23479616
references.citation: Biol Psychiatry. 2000 May 15;47(10):921-7
references.pmid: 10807965
references.citation: Oecologia. 1975 Dec;18(4):269-316
references.pmid: 28308918
references.citation: Trends Cogn Sci. 2009 Oct;13(10):429-38
references.pmid: 19748817
references.citation: Proc Natl Acad Sci U S A. 2007 May 22;104(21):9081-6
references.pmid: 17502598
references.citation: Curr Biol. 2004 Dec 29;14(24):R1038-9
references.pmid: 15620633
references.citation: Am J Physiol. 1995 Jul;269(1 Pt 2):R173-8
references.pmid: 7631890
references.citation: J Neurosci. 2001 Aug 15;21(16):6405-12
references.pmid: 11487664
references.citation: Philos Trans R Soc Lond B Biol Sci. 2009 Oct 12;364(1531):2849-65
references.pmid: 19720649
references.citation: J Biol Rhythms. 2005 Apr;20(2):168-77
references.pmid: 15834113
references.citation: Physiol Behav. 1991 Sep;50(3):583-8
references.pmid: 1801013
references.citation: Chronobiol Int. 2009 Apr;26(3):474-93
references.pmid: 19360491
references.citation: Int J Chronobiol. 1976;4(2):97-110
references.pmid: 1027738
references.citation: Sleep. 2007 Nov;30(11):1484-501
references.pmid: 18041481
references.citation: Proc Natl Acad Sci U S A. 2001 Nov 20;98(24):14027-32
references.pmid: 11717461
references.citation: J Physiol. 2012 Jul 1;590(13):3035-45
references.pmid: 22547633
references.citation: Nature. 2011 Jul 17;476(7358):92-5
references.pmid: 21765429
references.citation: J Biol Rhythms. 2005 Aug;20(4):326-38
references.pmid: 16077152