3- 4:Circadian rhythm 5- -- 12- 13: A circadian rhythm is a roughly 24-hour cycle in the biochemical, 14- physiological or behavioural processes of living entities, including -- 36- * [30]5 Biological clock in mammals 37: + [31]5.1 Determining the human circadian rhythm 38- + [32]5.2 Outside the "master clock" -- 51- 52: The earliest known account of a circadian rhythm dates from the 4th 53- century BC, when Androsthenes, a ship captain serving under -- 63- and changes in temperature.^[50][3] [51]Joseph Takahashi discovered the 64: genetic basis for the rodent circadian rhythm in 1994.^[52][4]^[53][5] 65- -- 72- adjusted to match the local time: 73: * The rhythm persists in constant conditions (for example, constant 74- dark) with a period of about 24 hours. The rationale for this -- 76- rhythms that are merely responses to external periodic cues. A 77: rhythm cannot be declared to be endogenous unless it has been 78- tested in conditions without external periodic input. 79: * The rhythm is temperature-compensated, i.e., it maintains the same 80- period over a range of temperatures. The rationale for this -- 85- coincidental. 86: * The rhythm can be reset by exposure to an external stimulus. The 87- rationale for this criterion is to distinguish circadian rhythms -- 124- just the three proteins of their central oscillator. This clock has 125: been shown to sustain a 22-hour rhythm over several days upon the 126- addition of [72]ATP. Previous explanations of the [73]prokaryotic -- 184- 185: The rhythm is linked to the light-dark cycle. Animals, including 186- humans, kept in total darkness for extended periods eventually function 187: with a [89]freerunning rhythm. Each "day", their sleep cycle is pushed 188- back or forward, depending on whether their [90]endogenous period is -- 199- will still have it when in an environment shielded from external cues, 200: but the rhythm is, of course, not entrained to the 24-hour light/dark 201: cycle in nature. The sleep-wake rhythm may, in these circumstances, 202- become out of phase with other circadian or [95]ultradian rhythms such -- 243- groups of [122]cells located in the [123]hypothalamus. Destruction of 244: the SCN results in the complete absence of a regular sleep-wake rhythm. 245- The SCN receives information about illumination through the eyes. The -- 250- tract, leading to the SCN. If cells from the SCN are removed and 251: cultured, they maintain their own rhythm in the absence of external 252- cues. -- 267- 268:[[135]edit] Determining the human circadian rhythm 269- 270- The classic phase markers for measuring the timing of a mammal's 271: circadian rhythm are 272- * melatonin secretion by the pineal gland and -- 323- response curve (PRC). Depending on the timing, light can advance or 324: delay the circadian rhythm. Both the PRC and the required 325- [155]illuminance vary from species to species and lower light levels -- 327- 328: Lighting levels that affect circadian rhythm in humans are higher than 329- the levels usually used in artificial lighting in homes. According to -- 337- It is thought that the direction of the light may have an effect on 338: entraining the circadian rhythm;^[160][24] light coming from above, 339- resembling an image of a bright sky, has greater effect than light -- 397- the day, a [181]power-nap, does not have any effect on normal circadian 398: rhythm, but can decrease stress and improve 399- productivity.^[182][33]^[183][34] -- 401- There are many health problems associated with disturbances of the 402: human circadian rhythm, such as [184]seasonal affective disorder (SAD), 403- [185]delayed sleep phase syndrome (DSPS) and other [186]circadian 404: rhythm disorders.^[187][35] Circadian rhythms also play a part in the 405- [188]reticular activating system, which is crucial for maintaining a -- 418- functioning of circadian rhythms. Recent research suggests that 419: circadian rhythm disturbances found in [199]bipolar disorder are 420- positively influenced by [200]lithium's effect on clock -- 426- cardiovascular disease [202][2] The suppression of melatonin production 427: associated with the disruption of the circadian rhythm may increase the 428- risk of developing cancer.^[203][38]^[204][39] -- 446- * [218]Circadian oscillator 447: * [219]Circadian rhythm sleep disorders 448- * [220]Cryptochrome -- 452- * [225]Jet lag 453: * [226]Light effects on circadian rhythm 454- * [227]PER1, [228]PER2, and [229]PER3, the period family genes -- 489- * Tomita J, Nakajima M, Kondo T, Iwasaki H (2005) No 490: transcription-translation feedback in circadian rhythm of KaiC 491- phosphorylation. Science 307: 251-254 -- 608- 29. [320]^ Dijk, Derk-Jan; Czeisler Charles (1994). "Paradoxical timing 609: of the circadian rhythm of sleep propensity serves to consolidate 610- sleep and wakefulness in humans". Neurosci Lett 166 (1): 63. -- 673- 674: * [373]Circadian rhythm at the [374]Open Directory Project 675- * Leloup J.C. (2009). "Circadian clocks and phosphorylation: Insights