Editing Phase response curve (section)

===Light===
[[Image:Human PRC.png|thumb|right|400px|alt=A typical Human Light PRC|The time shown on the ''x'' axis is vague: dawn{{snd}} mid-day{{snd}} dusk{{snd}} night{{snd}} dawn. These times do not refer to actual sun-up etc. nor to specific clock times. Each individual has their own circadian "clock" and chronotype, and dawn in the illustration refers to an individual's time of spontaneous awakening when well-rested and sleeping regularly. The PRC shows when a stimulus, in this case light to the eyes, will effect a change, an advance or a delay. The curve's highest point coincides with the subject's lowest body temperature.]]

Starting about two hours before an individual's regular bedtime, exposure of the eyes to light will delay the circadian phase, causing later wake-up time and later sleep onset. The delaying effect gets stronger as evening progresses; it is also dependent on the wavelength and [[illuminance]] ("brightness") of the light. The effect is small if indoor lighting is dim {{nobr|(< 3 lux).}}

About five hours after usual bedtime, coinciding with the body temperature trough (the lowest point of the core body temperature during sleep) the PRC peaks and the effect changes abruptly from phase delay to phase advance. Immediately after this peak, light exposure has its greatest phase-advancing effect, causing earlier wake-up and sleep onset. Again, illuminance greatly affects results; indoor light may be less than 500&nbsp;lux, while [[light therapy]] uses up to 10,000&nbsp;lux. The effect diminishes until about two hours after spontaneous wake-up time, when it reaches approximately zero.

During the period between two hours after usual wake-up time and two hours before usual bedtime, light exposure has little or no effect on circadian phase (slight effects generally cancelling each other out).

Another image of the PRC for light is here (Figure 1).<ref>{{Cite web |last=Staff |first=Sleep Review |date=2001-01-07 |title=Bringing Therapy to Light |url=https://sleepreviewmag.com/sleep-treatments/therapy-devices/light-therapy/bringing-therapy-to-light/ |access-date=2024-12-22 |website=Sleep Review |language=en-US}}</ref> Within that image, the explanatory text is 
* ''Delay region: evening light shifts sleepiness later'' and 
* ''Advance region: morning light shifts sleepiness earlier''.<ref>{{cite journal |vauthors = Kripke DF, Loving RT |title=Bringing Therapy to Light |journal=Sleep Review |date=2001 |issue=1 |url=http://www.sleepreviewmag.com/2001/01/bringing-therapy-to-light/}}</ref>

Light therapy, typically with a light box producing 10,000&nbsp;lux at a prescribed distance, can be used in the evening to delay or in the morning to advance an individual's sleep timing. Because losing sleep to obtain bright light exposure is considered undesirable by most people, and because it is very difficult to estimate exactly when the greatest effect (the PRC peak) will occur in an individual, the treatment is usually applied daily just prior to bedtime (to achieve phase delay), or just after spontaneous awakening (to achieve phase advance).

In addition to its use in the adjustment of circadian rhythms, light therapy is used as treatment for several [[Mood disorder|affective disorders]] including [[seasonal affective disorder]] (SAD).<ref>{{cite journal |vauthors = Walsh J, Atkinson LA, Corlett SA, Lall GS |title=An insight into light as a chronobiological therapy |journal=ChronoPhysiology and Therapy |date=2014 |volume=4 |pages=79–85 |doi=10.2147/CPT.S56589 |url=https://www.researchgate.net/publication/266673182 |access-date=31 May 2015 |doi-access=free}}</ref>

In 2002 [[Brown University]] researchers led by David Berson announced the discovery of special cells in the [[human eye]], ipRGCs ([[intrinsically photosensitive retinal ganglion cells]]),<ref>[http://www.brown.edu/Administration/News_Bureau/2004-05/04-076.html Brown Scientists Uncover Inner Workings of Rare Eye Cells].</ref> which, many researchers now believe, control the light entrainment effect of the phase response curve. In the human eye, the ipRGCs have the greatest response to light in the 460–480&nbsp;nm (blue) range. In one experiment, 400&nbsp;lux of blue light produced the same effects as 10,000&nbsp;lux of white light from a fluorescent source.<ref>{{cite journal | vauthors = Glickman G, Byrne B, Pineda C, Hauck WW, Brainard GC | title = Light therapy for seasonal affective disorder with blue narrow-band light-emitting diodes (LEDs) | journal = Biological Psychiatry | volume = 59 | issue = 6 | pages = 502–7 | date = March 2006 | pmid = 16165105 | doi = 10.1016/j.biopsych.2005.07.006 | s2cid = 42586876 | url = https://jdc.jefferson.edu/cgi/viewcontent.cgi?article=1002&context=jmbcimfp | url-access = subscription }}</ref> A theory of spectral opponency, in which the addition of other spectral colors renders blue light less effective for circadian phototransduction, was supported by research reported in 2005.<ref>{{cite journal | vauthors = Figueiro MG, Bullough JD, Bierman A, Rea MS | title = Demonstration of additivity failure in human circadian phototransduction | journal = Neuro Endocrinology Letters | volume = 26 | issue = 5 | pages = 493–498 | date = October 2005 | pmid = 16264413 }}</ref>