Editing Phase response curve (section)

==In circadian rhythms==
[[Image:PRC-Light+Mel.png|thumb|right|Phase response curves for light and for melatonin administration]]
In humans and animals, there is a regulatory system that governs the phase relationship of an organism's internal [[circadian clock]] to a regular periodicity in the external environment (usually governed by the solar day). In most organisms, a stable phase relationship is desired, though in some cases the desired phase will vary by season, especially among mammals with seasonal mating habits.

In [[circadian rhythm]] research, a PRC illustrates the relationship between a [[chronobiotic]]'s time of administration (relative to the internal circadian clock) and the magnitude of the treatment's effect on circadian phase. Specifically, a PRC is a graph showing, by convention, time of the subject's endogenous day along the ''x''-axis and the amount of the phase shift (in hours) along the ''y''-axis. Each curve has one peak and one trough in each 24-hour cycle. Relative circadian time is plotted against phase-shift magnitude. The treatment is usually narrowly specified as a set intensity and colour and duration of light exposure to the retina and skin, or a set dose and formulation of melatonin.

These curves are often consulted in the therapeutic setting. Normally, the body's various physiological rhythms will be synchronized within an individual organism (human or animal), usually with respect to a master biological clock. Of particular importance is the sleep–wake cycle. Various sleep disorders and externals stresses (such as [[jet lag]]) can interfere with this. Humans with [[non-24-hour sleep–wake disorder]] often experience an inability to maintain a consistent internal clock. Extreme [[chronotype]]s usually maintain a consistent clock, but find that their natural clock does not align with the expectations of their social environment. PRC curves provide a starting point for therapeutic intervention. The two common treatments used to shift the timing of sleep are [[light therapy]], directed at the eyes, and administration of the hormone [[melatonin]], usually taken orally. Either or both can be used daily. The phase adjustment is generally cumulative with consecutive daily administrations, and — at least partially — additive with concurrent administrations of distinct treatments. If the underlying disturbance is stable in nature, ongoing daily intervention is usually required. For jet lag, the intervention serves mainly to accelerate natural alignment, and ceases once desired alignment is achieved.

Note that phase response curves from the experimental setting are usually aggregates of the test population, that there can be mild or significant variation within the test population, that individuals with sleep disorders often respond atypically, and that the formulation of the chronobiotic might be specific to the experimental setting and not generally available in clinical practice (e.g. for melatonin, one sustained-release formulation might differ in its release rate as compared to another); also, while the magnitude is dose-dependent,<ref>{{cite journal | vauthors = Burgess HJ, Revell VL, Eastman CI | title = A three pulse phase response curve to three milligrams of melatonin in humans | journal = The Journal of Physiology | volume = 586 | issue = 2 | pages = 639–47 | date = January 2008 | pmid = 18006583 | pmc = 2375577 | doi = 10.1113/jphysiol.2007.143180 }}</ref> not all PRC graphs cover a range of doses. The discussions below are restricted to the PRCs for the light and melatonin in humans.

===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>

===Melatonin===

The phase response curve for melatonin is roughly twelve hours out of phase with the phase response curve for light.<ref>{{cite journal | vauthors = Lewy AJ, Ahmed S, Jackson JM, Sack RL | title = Melatonin shifts human circadian rhythms according to a phase-response curve | journal = Chronobiology International | volume = 9 | issue = 5 | pages = 380–92 | date = October 1992 | pmid = 1394610 | doi = 10.3109/07420529209064550 }}</ref> At spontaneous wake-up time, [[exogenous]] (externally administered) melatonin has a slight phase-delaying effect. The amount of phase-delay increases until about eight hours after wake-up time, when the effect swings abruptly from strong phase delay to strong phase advance. The phase-advance effect diminishes as the day goes on until it reaches zero about bedtime. From usual bedtime until wake-up time, exogenous melatonin has no effect on circadian phase.<ref>{{cite journal | vauthors = Burgess HJ, Revell VL, Eastman CI | title = A three pulse phase response curve to three milligrams of melatonin in humans | journal = The Journal of Physiology | volume = 586 | issue = 2 | pages = 639–47 | date = January 2008 | pmid = 18006583 | pmc = 2375577 | doi = 10.1113/jphysiol.2007.143180 }}</ref><ref>{{cite journal | vauthors = Lewy A | title = Clinical implications of the melatonin phase response curve | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 95 | issue = 7 | pages = 3158–60 | date = July 2010 | pmid = 20610608 | pmc = 2928905 | doi = 10.1210/jc.2010-1031 }}</ref>

The human body produces its own ([[endogenous]]) melatonin starting about two hours before bedtime, provided the lighting is dim. This is known as ''dim-light melatonin onset'', DLMO.<ref>{{cite journal | vauthors = Sletten TL, Vincenzi S, Redman JR, Lockley SW, Rajaratnam SM | title = Timing of sleep and its relationship with the endogenous melatonin rhythm | journal = Frontiers in Neurology | volume = 1 | pages = 137 | date = 2010 | pmid = 21188265 | pmc = 3008942 | doi = 10.3389/fneur.2010.00137 | doi-access = free }}</ref> This stimulates the phase-advance portion of the PRC and helps keep the body on a regular sleep-wake schedule. It also helps prepare the body for sleep.

Administration of melatonin at any time may have a mild [[hypnotic]] (sleep-inducing) effect. The expected effect on sleep phase timing, if any, is predicted by the PRC.

===Additive effects===
In a 2006 study Victoria L. Revell ''et al.'' showed that a combination of morning bright light and afternoon melatonin, both timed to phase advance according to the respective PRCs, produce a larger phase advance shift than bright light alone, for a total of up to 2{{frac|1|2}} hours over the course of 3 days. All times are approximate and vary from one individual to another. In particular, there is no convenient way to accurately determine the times of the peaks and zero-crossings of these curves in an individual. Administration of light or melatonin close to the time at which the effect is expected to change sense abruptly may, if the changeover time is not accurately known, produce an opposite effect to that desired.<ref>{{cite journal | vauthors = Revell VL, Burgess HJ, Gazda CJ, Smith MR, Fogg LF, Eastman CI | title = Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 91 | issue = 1 | pages = 54–9 | date = January 2006 | pmid = 16263827 | pmc = 3841985 | doi = 10.1210/jc.2005-1009 }}</ref>

=== Exercise ===
In a 2019 study Shawn D. Youngstedt ''et al.'', showed that in humans "Exercise elicits circadian phase‐shifting effects, but additional information is needed. [...] Significant phase–response curves were established for [[6-Hydroxymelatonin|aMT6(melatonin derivative)]] onset and acrophase with large phase delays from 7:00 pm to 10:00 pm and large phase advances at both 7:00 am and from 1:00 pm to 4:00 pm"<ref>{{cite journal | vauthors = Youngstedt SD, Elliott JA, Kripke DF | title = Human circadian phase-response curves for exercise | journal = The Journal of Physiology | volume = 597 | issue = 8 | pages = 2253–2268 | date = April 2019 | pmid = 30784068 | pmc = 6462487 | doi = 10.1113/JP276943 }}</ref>

=== Origin ===
The first published usage of the term "phase response curve" was in 1960 by [[Patricia DeCoursey]]. The "daily" activity rhythms of her [[flying squirrel]]s, kept in constant darkness, responded to pulses of light exposure. The response varied according to the time of day – that is, the animals' subjective "day" – when light was administered. When DeCoursey plotted all her data relating the quantity and direction (advance or delay) of phase-shift on a single curve, she created the PRC. It has since been a standard tool in the study of biological rhythms.<ref>{{cite web |title=Clock Tutorial #3c - Darwin On Time |url=http://scienceblogs.com/clock/2007/08/clock_tutorial_3c_darwin_on_ti.php |access-date=2007-11-03 | vauthors = Zivkovic B |year=2007 |work=A Blog Around the Clock |publisher=ScienceBlogs LLC |archive-url=https://web.archive.org/web/20120519024856/http://scienceblogs.com/clock/2007/08/clock_tutorial_3c_darwin_on_ti.php |archive-date=2012-05-19 |quote=[The PRC is] the single most important methodological tool in the study of all [[biological rhythm]]s. |url-status=dead }}</ref>