question: Why does it take a few minutes for your eyes to adjust to darkness?
comment: [Rhodopsin]( URL_0 ) is a protein photopigment used by the rod cells of the retina to detect gray-scale brightness and contrast, and is the key to adaptive night vision. Intense light causes the protein to decompose reducing its sensitivity in bright light (when we rely more heavily on our color-sensing cone cells anyway). Darkness allows the protein to regenerate in a process called “dark adaptation”, allowing the eye to see in low light conditions, but with reduced color discernment. This process allows us to be able to see fairly effectively in both bright and dim light, but it takes time for our vision to adjust to sudden shifts in brightness in either direction.
passages: Section::::In circadian rhythms.:Light.
  
  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 in dim indoor lighting.
   The retina has a static contrast ratio of around 100:1 (about 6.5 f-stops). As soon as the eye moves rapidly to acquire a target (saccades), it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal rod photoreceptors is 80% complete in thirty minutes. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by retinal disease, poor vascular circulation and high altitude exposure. 
   Darkness
  
  Darkness, the polar opposite of brightness, is understood as a lack of illumination or an absence of visible light.
  
  Human vision is unable to distinguish color in conditions of either high brightness or high darkness. In conditions with insufficient light levels, color perception ranges from achromatic to ultimately black.
  
  The emotional response to darkness has generated metaphorical usages of the term in many cultures.
  
  Referring to a time of day, complete darkness occurs when the Sun is more than 18° below the horizon, without the effects of twilight on the night sky.
  
  Section::::Scientific.
  
  Section::::Scientific.:Perception.
   The eye takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and becomes 10,000 to 1,000,000 times more sensitive than at full daylight. In this process, the eye's perception of color changes as well (this is called the Purkinje effect). However, it takes approximately five minutes for the eye to adapt from darkness to bright sunlight. This is due to cones obtaining more sensitivity when first entering the dark for the first five minutes but the rods taking over after five or more minutes. Cone cells are able to regain maximum retinal sensitivity in 9–10 minutes of darkness whereas rods require 30–45 minutes to do so.
   Before nautical dawn and after nautical dusk, sailors cannot navigate via the horizon at sea as they cannot clearly see the horizon. At nautical dawn and nautical dusk, the human eye finds it difficult, if not impossible, to discern traces of illumination near the sunset or sunrise point of the horizon ("first light” after nautical dawn but before civil dawn and "nightfall" after civil dusk but before nautical dusk).
   Section::::Works.:"Night Vision".
   Complete darkness or astronomical night is the period between astronomical dusk and astronomical dawn when the Sun is between 18 and 90 degrees below the horizon and does not illuminate the sky. As seen from latitudes between 48.5° and 66.5° north or south of the Equator, complete darkness does not occur around the summer solstice because although the Sun sets, it is never more than 18° below the horizon at lower culmination.
   The retinal must diffuse from the vision cell, out of the eye, and circulate via the blood to the liver where it is regenerated. In bright light conditions, most of the retinal is not in the photoreceptors, but is outside of the eye. It takes about 45 minutes of dark for "all" of the photoreceptor proteins to be recharged with active retinal, but most of the night vision adaptation occurs within the first five minutes in the dark. Adaptation results in maximum sensitivity to light. In dark conditions only the rod cells have enough sensitivity to respond and to trigger vision.
   Exposure to monochromatic light at the wavelengths of 460 nm and 550 nm on two control groups yielded results showing decreased sleepiness at 460 nm tested over two groups and a control group. Additionally, in the same study but testing thermoregulation and heart rate researchers found significantly increased heart rate in 460 nm light over the course of a 1.5 hour exposure period.
   Circadian rhythm disruption is primarily caused by the wrong timing of light in reference to the circadian phase. It can also be affected by too much light, too little light, or incorrect spectral composition of light. This effect is driven by stimulus (or lack of stimulus) to photosensitive ganglion cells in the retina. The "time of day", the circadian phase, is signalled to the pineal gland, the body’s photometer, by the suprachiasmatic nucleus. Bright light in the evening or in the early morning shifts the phase of the production of melatonin (see phase response curve). An out-of-sync melatonin rhythm can worsen cardiac arrhythmias and increase oxidized lipids in the ischemic heart. Melatonin also reduces superoxide production and myeloperoxide (an enzyme in neutrophils which produces hypochlorous acid) during ischemia-reperfusion.
   Although motion and images are extremely important regarding adaptation, the most important adaptation is adjusting to brightness levels. On entering a dark room or a very brightly lit room it takes a little while to adjust to the different levels. Adjusting to brightness levels allows mammals to detect changes in their surrounding. This is called dark adaptation.
  
  Section::::Auditory.
   All of the mechanisms of light-effected entrainment are not yet fully known, however numerous studies have demonstrated the effectiveness of light entrainment to the day/night cycle. Studies have shown that the timing of exposure to light influences entrainment; as seen on the phase response curve for light for a given species. In diurnal (day-active) species, exposure to light soon after wakening advances the circadian rhythm, whereas exposure before sleeping delays the rhythm. An advance means that the individual will tend to wake up earlier on the following day(s). A delay, caused by light exposure before sleeping, means that the individual will tend to wake up later on the following day(s).
   Most animals and other organisms have "built-in clocks" in their brains that regulate the timing of biological processes and daily behavior. These "clocks" are known as circadian rhythms. They allow maintenance of these processes and behaviors relative to the 24-hour day/night cycle in nature. Although these rhythms are maintained by the individual organisms, their length does vary somewhat individually. Therefore, they must, either continually or repeatedly, be reset to synchronize with nature's cycle. In order to maintain synchronization ("entrainment") to 24 hours, external factors must play some role. The human circadian rhythm occurs typically in accordance with nature's cycle. The average activity rhythm cycle is 24.18 hours in adulthood but is shortened as age increases. One of the various factors that influence this entrainment is light exposure to the eyes. When an organism is exposed to a specific wavelength of light stimulus at certain times throughout the day, the hormone melatonin is suppressed, or prevented from being secreted by the pineal gland.
   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 lux while light therapy uses up to 10,000 lux. The effect diminishes until about two hours after spontaneous wake-up time, when it reaches approximately zero.
   The perception of darkness differs from the mere absence of light due to the effects of after images on perception. In perceiving, the eye is active, and the part of the retina that is unstimulated produces a complementary afterimage.
  
  Section::::Scientific.:Physics.
   Section::::Biblical account.
  
  The oldest biblical reference to the crucifixion darkness is found in the Gospel of Mark, written around the year 70. In its account of the crucifixion, on the eve of Passover, it says that after Jesus was crucified at nine in the morning; darkness fell over all the land, or all the world ( can mean either) from around noon ("the sixth hour") until 3 o'clock ("the ninth hour"). It adds, immediately after the death of Jesus, that "the curtain of the temple was torn in two, from top to bottom".
   Blessed Anna Maria Taigi (1769–1837) is the most known seer of the Three Days of Darkness and describes the event in this way:
   Section::::Mechanism.
  
  Light first passes into a mammal's system through the retina, then takes one of two paths: the light gets collected by rod cells and cone cells and the retinal ganglion cells (RGCs), or it is directly collected by these RGCs.
   In Arctic and Antarctic latitudes in wintertime, the polar night only rarely produces complete darkness for 24 hours each day. This can occur only at locations within 5.5 degrees of latitude of the Pole, and there only on dates close to the winter solstice. At all other latitudes and dates, the polar night includes a daily period of twilight, when the Sun is not far below the horizon. Around winter solstice, when the solar declination changes slowly, complete darkness lasts several weeks at the Pole itself, e.g., from May 11 to July 31 at Amundsen–Scott South Pole Station. North Pole has the experience of this from November 12 to January 30.
   Researcher Mark Changizi of Rensselaer Polytechnic Institute in New York has a more imaginative take on optical illusions, saying that they are due to a neural lag which most humans experience while awake. When light hits the retina, about one-tenth of a second goes by before the brain translates the signal into a visual perception of the world. Scientists have known of the lag, yet they have debated how humans compensate, with some proposing that our motor system somehow modifies our movements to offset the delay.
   Babylonian scholar of the fourth century. Once on a cloudy Friday, he thought that the sun had set, and at once recited the Sabbath prayer. Subsequently the skies cleared, and he discovered his mistake. Upon his seeking rabbinic guidance, Abaye declared that he could resume his weekday work.
  
  Section::::Dimi of Nehardea.
   Though oscillators in the skin respond to light, a systemic influence has not been proven. In addition, many oscillators, such as liver cells, for example, have been shown to respond to inputs other than light, such as feeding.
  
  Section::::Light and the biological clock.
  
  Light resets the biological clock in accordance with the phase response curve (PRC). Depending on the timing, light can advance or delay the circadian rhythm. Both the PRC and the required illuminance vary from species to species and lower light levels are required to reset the clocks in nocturnal rodents than in humans.
  
  Section::::Enforced longer cycles.
   The hormones cortisol and melatonin are effected by the signals light sends through the body's nervous system. These hormones help regulate blood sugar to give the body the appropriate amount of energy that is required throughout the day. Cortisol is levels are high upon waking and gradual decrease over the course of the day, melatonin levels are high when the body is entering and exiting a sleeping status and are very low over the course of waking hours. The earth's natural light-dark cycle is the basis for the release of these hormones.
   Nocturnal animals alter the timing of their foraging based on the level of light – avoiding feeding when the moonlight is bright as this is when predation risk is highest.
   Section::::In marine animals.:Mechanism.:Matching light intensity and wavelength.
  
  At night, nocturnal organisms match both the wavelength and the light intensity of their bioluminescence to that of the down-welling moonlight and direct it downward as they swim, to help them remain unnoticed by any observers below.
  