Editing Chronobiology (section)

==Psychological impact of light exposure==
Previous studies in circadian biology have established that exposure to light during abnormal hours leads to [[sleep deprivation]] and disruption of the circadian system, which affect mood and [[Cognition|cognitive functioning]]. While this indirect relationship had been corroborated, not much work had been done to examine whether there was a direct relationship between irregular light exposure, aberrant mood, cognitive function, normal sleep patterns and circadian oscillations. In a study published in 2012, the Hattar Laboratory was able to show that deviant light cycles directly induce [[Depression (mood)|depression]]-like symptoms and lead to impaired learning in mice, independent of [[sleep]] and circadian oscillations.<ref name=Dulcis>{{cite journal|last1=Dulcis|first1=Davide|last2=Jamshidi|first2=Pouya|last3=Leutgeb|first3=Stefan|last4=Spitzer|first4=Nicholas C.|title=Neurotransmitter Switching in the Adult Brain Regulates Behavior|journal=Science|date=26 April 2013|volume=340|issue=6131|pages=449–453|bibcode=2013Sci...340..449D|doi=10.1126/science.1234152|pmid=23620046|s2cid=44911091}}</ref>

===Effect on mood===
ipRGCs project to areas of the brain that are important for regulating circadian rhythmicity and sleep, most notably the [[Suprachiasmatic nucleus|SCN]], subparaventricular nucleus, and the ventrolateral preoptic area. In addition, ipRGCs transmit information to many areas in the [[limbic system]], which is strongly tied to emotion and memory. To examine the relationship between deviant light exposure and behavior, Hattar and his colleagues studied mice exposed to alternating 3.5-hour light and dark periods (T7 mice) and compared them with mice exposed to alternating 12-hour light and dark periods (T24 mice).  Compared to a T24 cycle, the T7 mice got the same amount of total sleep and their circadian expression of [[PER2]], an element of the SCN pacemaker, was not disrupted. Through the T7 cycle, the mice were exposed to light at all circadian phases. Light pulses presented at night lead to expression of the transcription factor [[c-Fos]] in the [[amygdala]], [[habenula|lateral habenula]], and subparaventricular nucleus further implicating light's possible influence on mood and other cognitive functions.<ref name=Masana>{{cite journal|last1=Masana|first1=MI|title=Light-induced c-fos mRNA expression in the suprachiasmatic nucleus and the retina of C3H/HeN mice.|journal=Molecular Brain Research|date= December 1996|volume=42|issue=2|pages=193–201|doi=10.1016/s0169-328x(96)00031-9|pmid=9013774}}</ref>

Mice subjected to the T7 cycle exhibited depression-like symptoms, exhibiting decreased preference for [[sucrose]] (sucrose anhedonia) and exhibiting more immobility than their T24 counterparts in the [[behavioural despair test|forced swim test (FST)]].  Additionally, T7 mice maintained rhythmicity in serum [[corticosterone]], however the levels were elevated compared to the T24 mice, a trend that is associated with depression. Chronic administration of the antidepressant [[Fluoxetine]] lowered corticosterone levels in T7 mice and reduced depression-like behavior while leaving their circadian rhythms unaffected.<ref name=Dulcis />

===Effect on learning===
The [[hippocampus]] is a structure in the limbic system that receives projections from ipRGCs. It is required for the consolidation of [[short-term memory|short-term memories]] into [[long-term memory|long-term memories]] as well as spatial orientation and navigation. Depression and heightened serum corticosterone levels are linked to impaired hippocampal learning. Hattar and his team analyzed the T7 mice in the [[Morris water navigation task|Morris water maze (MWM)]], a spatial learning task that places a mouse in a small pool of water and tests the mouse's ability to locate and remember the location of a rescue platform located just below the waterline.  Compared to the T24 mice, the T7 mice took longer to find the platform in subsequent trials and did not exhibit a preference for the quadrant containing the platform. In addition, T7 mice exhibited impaired hippocampal [[long-term potentiation]] (LTP) when subjected to [[Theta Burst Stimulation|theta burst stimulation]] (TBS). Recognition memory was also affected, with T7 mice failing to show preference for novel objects in the novel object recognition test.<ref name=Sauer>{{cite journal|last1=Sauer|first1=Jonas-Frederic|title=Impaired fast-spiking interneuron function in a genetic mouse model of depression|journal=eLife|date=3 March 2015|volume=4|doi=10.7554/elife.04979|pmid=25735038|pmc=4374525 |doi-access=free }}</ref>

===Necessity of ipRGCs===
Mice without (Opn4<sup>aDTA/aDTA</sup> mice) are not susceptible to the negative effects of an aberrant light cycle, indicating that light information transmitted through these cells plays an important role in regulation of mood and cognitive functions such as learning and memory.<ref name=Monteggia>{{cite journal|last1=Monteggia|first1=Lisa|author-link1=Lisa Monteggia|last2=Kavalali|first2=E. T.|title=Circadian rhythms: Depression brought to light|journal=Nature|volume=491|issue=7425|pages=537–538|doi=10.1038/nature11752|pmid=23151474|year=2012|bibcode=2012Natur.491..537M|s2cid=4391543}}</ref>