Why does being out in the sun make you tired?
In: Biology
For most people, it doesn’t. Indeed some people with seasonal affective disorder find extra sunlight to be invigorating.
There are some people who have suffered demyelinated nerve damage ( MS, ALS, and a handful of other medical conditions ) and who do feel fatigue when exposed to sunlight. The damaged optic nerve simply does not have the bandwidth to handle all the data that the eyeball is trying to transmit. The result is pain, fatigue, and irritability.
If the light is really bright ( like midday in the Sahara desert ) even a healthy person may find that the light makes them fatigued. Again, it is the same issue of bandwidth.
In either case, the connection between inadequate bandwidth and fatigue is not well understood. The best theory that I have heard borrows the concept of ‘crosstalk’ from the field of electronics. IOW, messages ‘jump’ from one nerve to another. This causes unexpected messages in nerves that sense pain, and in nerves that regulate sleep.
EDIT: I see that someone downvoted this post. If you disagree with it, please be good enough to reply and say why you disagree. Being one of those people with one of the aforementioned ‘other medical conditions’, I’d really like to hear if I am mistaken.
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