Photic information entrains a circadian pacemaker located in the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus to environmental light/dark cycles. To determine whether light regulates c-fos gene expression in the SCN, we have measured c-fos mRNA levels in the SCN of the golden hamster. We report that, during the subjective night, light causes a rapid increase in levels of c-fos mRNA in the SCN. Light pulses of 5 min duration are sufficient to induce c-fos mRNA, and the highest mRNA levels occur 30 min following the onset of light. The minimum level of illumination required to induce an increase in c-fos mRNA is indistinguishable from the minimum irradiance that produces a phase shift in the hamster's circadian rhythm of activity. In addition, the induction of c-fos mRNA in the SCN by light is itself under circadian regulation. Light induction of c-fos mRNA occurs only during the subjective night, at circadian times when photic phase shifting of activity occurs. Taken together, these data suggest that c-fos may be a molecular component of the photic pathway for entrainment of mammalian circadian rhythms.
If elevated c-fos is connected to the creation of addictive neural assemblies in the brain (briefly discussed in the preceding post), then being exposed to light during what should be our sleeping period might increase the possibility of becoming addicted to whatever pleasurable thing we are doing while up at night.
Why do casinos keep the lights on constantly? Is it just so that casino patrons don't notice how much time they've been inside gambling, or have the casinos observed an increase in gambling addiction-like behavior as a result of their exposing their patrons to light when the patrons ought to be in bed?
Why are night owls more prone to addictive disorders than early risers? (https://www.ncbi.nlm.nih.gov/pubmed/26250146) Is it because they are engaged in their addictions when their bodies should be asleep?
Being an evening person, rather than a morning person, was recently found to be linked to a higher risk of becoming addicted to smartphones in German adolescents. (https://www.ncbi.nlm.nih.gov/pubmed/27499228) Is that because the evening people are more likely to have a smartphone shining in their faces when they should be asleep?
All very interesting. It almost makes me want to go to bed earlier. Actually, why don't I? I can read in the early morning just as easily as in the late evening. Perhaps it's time to make my bedtime firm.