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In response to light exposure, alertness levels can increase as a result of suppression of melatonin secretion. [3] [6] A linear relationship has been found between alerting effects of light and activation in the posterior hypothalamus. [3] [24] Disruption of circadian rhythm as a result of light also produces changes in metabolism. [4]
One variable tested in the bunker experiment was whether light intensity affected the period of the circadian rhythm. To examine this, Aschoff and colleagues manipulated the intensity of light from outside the bunker, exposing subjects to dim light (40 lux), then light of high intensity (200 lux) when the subjects chose to turn on the lights.
1986 – Discovered that light resets the human circadian pacemaker, not social cues or sleep-wake cycles as previously thought. [27] 1989 – Characterized the profound sensitivity of the human circadian clock to light. [28] 1995 – Discovered that blind people can still retain sleep rhythms if their eyes remain intact. [29]
Light therapy can also be used to treat circadian rhythm sleep disorders. These disorders are caused by discrepancies between one’s circadian rhythm and the light/dark cycle of the environment. People with a sleep disorder experience insomnia or hypersomnia .
An example is the interaction between circadian rhythms and environmental cues, such as light and temperature. Entrainment helps organisms adapt their bodily processes with the timing of a changing environment. [1] For example, entrainment is manifested during travel between time zones, hence why humans experience jet lag.
Early research into circadian rhythms demonstrated that, when humans are without zeitgebers, or in constant lighting conditions, they have a "free running" period of 24.9 hours. [7] In this experiment, humans were in an underground, sound-proof bunker where the lights were continuously on.
In vertebrates, the master circadian clock is contained within the suprachiasmatic nucleus (SCN), a bilateral nerve cluster of about 20,000 neurons. [10] [11] The SCN itself is located in the hypothalamus, a small region of the brain situated directly above the optic chiasm, where it receives input from specialized photosensitive ganglion cells in the retina via the retinohypothalamic tract.
A circadian rhythm (/ s ər ˈ k eɪ d i ə n /), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours.Circadian rhythms can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (is entrained by the environment).