Normal rhythm
A circadian rhythm is an entrainable, endogenous, biological activity that has a period of roughly twenty-four hours. This internal time-keeping mechanism is centralized in the suprachiasmatic nucleus (SCN) of humans, and allows for the internal physiological mechanisms underlying sleep and alertness to become synchronized to external environmental cues, like the light-dark cycle. The SCN also sends signals to peripheral clocks in other organs, like the liver, to control processes such as glucose metabolism. Although these rhythms will persist in constant light or dark conditions, different Zeitgebers (time givers such as the light-dark cycle) give context to the clock and allow it to entrain and regulate expression of physiological processes to adjust to the changing environment. Genes that help control light-induced entrainment include positive regulators BMAL1 and CLOCK and negative regulators PER1 and CRY. A full circadian cycle can be described as a twenty-four hour circadian day, where circadian time zero (CT 0) marks the beginning of a subjective day for an organism and CT 12 marks the start of subjective night.Humans with regular circadian function have been shown to maintain regular sleep schedules, regulate daily rhythms in hormone secretion, and sustain oscillations in core body temperature. Even in the absence of Zeitgebers, humans will continue to maintain a roughly 24-hour rhythm in these biological activities.
Halo nevus (also known as "Leukoderma acquisitum centrifugum," "Perinevoid vitiligo," and "Sutton nevus": 689 ) is a mole that is surrounded by a depigmented ring or halo. Presentation
Halo nevi are also known as Suttons nevi, or leukoderma acquisitum centrifugum. Halo nevi are named such because they are a mole (nevus) that is surrounded by an area of depigmentation that resembles a halo. Halo nevi are associated with vitiligo. Sometimes the pale (hypopigmented) areas will spontaneously regress, and pigment returns. Causes
The formation of a halo surrounding a nevi is believed to occur when certain white blood cells called CD8+ T lymphocytes destroy the pigment-producing cells of the skin (melanocytes). The cause for the attack is unknown. Diagnosis
Treatment
As halo nevi are only of cosmetic significance, no treatment is required, and patients will be asymptomatic. Although halo nevi are harmless, it is important to monitor the lesion on regular basis. Watch out for any changes in appearance of existing or new halo nevi. If there is any change in appearance or is associated with pain, itch, and infection, a doctor should be consulted immediately to exclude the possibility of melanoma. Epidemiology
Halo nevi are estimated to be present in approximately 1% of the general population, and are found to be more prevalent in people with vitiligo, malignant melanoma, or Turner syndrome. All races and sexes are equally susceptible to this disease, although a familial tendency has been reported. The average age of onset is in a persons teenage years. See also
Nevus
List of cutaneous conditions

References


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