|MadSci Network: Medicine|
There is information about why flu/cold symptoms get worse "when the sun goes down." Lets start with the circadian rhythm (which is the bad guy or good guy) depending on your health at the time.
The body's biological time clock is initiated by the light or lack of light hitting what is known as the suprachiasmatic nucleus or (SCN) located in the retina. So to answer one of your questions - yes light does effect your symptoms in an indirect/direct way depending on how you view it (no pun intended).
Starting about three - four hours before you get ready for bed (providing you are on a "normal" day schedule) your body temp begins to drop. It drops sharply (about 4-5 degrees) about an hour before you go to sleep - which probably is one of the most significant occurrence that effects the way you feel - from my reading. As your temp drops your melatonin increases, stomach acid increases, blood pressure goes down and your pain threshold reaches it's maximum.
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