One additional factor to consider in Paul's ME/CFS recovery through moving to Costa Rica is that, instead of, or addition to, vitamin D,
it may be that the increased light levels in Costa Rica are playing a role.
If an individual suffers from
seasonal affective disorder (SAD), also known as winter depression, then the lack of light in dark northern latitude winters can cause the
symptoms of SAD, which result from reduced blood flow to certain regions of the brain. SAD is present in a subset of chronic fatigue syndrome patients, and some researchers think that SAD and ME/CFS may share common causes.
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As the average light level diminishes in dark northern latitude winters, then brain blood flow also diminishes in people with SAD. Reduce blood flow is already found in ME/CFS too, so perhaps ME/CFS is worsened when brain blood flow is reduced through SAD, in dark winters.
By contrast, once the ambient light shining into the eyes exceeds a certain daily average level, brain blood flow is restored, and the symptoms of SAD are eliminated. There are in fact specialized light receptors on the retina of the eye that sense average ambient light levels, and these receptors determine brain blood flow. These receptors are called
photosensitive ganglion cells, and they are different to the light receptor cells of the eye used in normal vision (rod and cone receptors). Photosensitive ganglion cells do not produce any visual output; their function is just to determine average ambient light levels.
People with SAD often treat their condition by placing bright lights in front of them during the winter. You need to expose your eyes to a light level of around 10,000 lux for about 30 minutes a day in order to mitigate or eliminate SAD symptoms.
However, moving to a very sunny location like Costa Rica is the ultimate SAD treatment.