Typical advice doctor gives you when you're sick, particularly with common cold and flu is:

Avoid getting into direct sunlight.

My anecdotal experience is that indeed when exposed to sunlight when sick (eg. when waiting for bus home) makes me feel very weak and raises the urge to seek shadow, as if I was some kind of zombie. I observed the behavior in others too.

The effect is quite fast for it to be dehydration or something like that, so what is it exactly?

  • Fever+sun = bad Jun 9, 2016 at 14:39
  • @Blue_Elephant Yes, that was what doctor suggested and I observed. But why would that be the case? Jun 9, 2016 at 14:53
  • Zato Because your body needs to maintain temperature. One degree more is already bad, and sun will heat it up. That'll get worse. Jun 9, 2016 at 15:08
  • @Blue_Elephant That doesn't explain feeling better the shadow in burning-hot interior of the public traffic, much hotter than sunlight feels. Jun 9, 2016 at 15:09
  • 1
    @Blue_Elephant - If you could put your comments in an answer with references, it would make a good answer. As it is, you are answering in comments which can be deleted at any time and is against general SE policy.
    – JohnP
    Jun 9, 2016 at 15:47

1 Answer 1


First of all, one needs to know its body needs to maintain its internal temperature, which is about 37° if I remember right. This is a process called homeostasis .

When you're sick, fever symptoms can occur, and your body heats up.

Now, if you're exposed to the sun too, you are causing a temperature elevation in your body.

More specifically, to answer to your secondary as to why staying in a hot interior is still better than direct exposition to sun, it's because your body temperature is regulated by contracting or dilating your external vessels (those who are close to the skin). When you are directly exposed to a heat source, the exposed zone is adapting by playing on dilatation or contraction (I can't remember which one for heat or cold) of the external vessels. Too much of this can cause malaise.

This is a part of human thermoregulation process.

Edit: As to references into the literature on heat exposure effects, there are quite a bit, here are some quotes :

Knochel 1974, review

-Heat exhaustion [...] due to predominant water depletion [...]. Symptoms (of hypertonic dehydration, ndlr) include fatigue, anxiety, weakness [...].
-Examining the response of resting normal subjects to hyperthermia [...] showed that elevation of skin temperature [...] was associated with a rise of arterial blood temperature [...] up to a 125% increase of cardiac output [...].

See also Hales 1998 for related physiology that could lead to heat stroke.

I also agree that, as some comments pointed out, normal sunlight exposure shouldn't normally cause malaise, but I was referring to hot sunlight exposure, which, with conjunction with fever, is prone to get the body to abnormally high temperature.

  • I seriously doubt that exposure to normal levels of sunlight will cause malaise in anyone other than people sensitive to sun exposure. -1 until you produce evidence supporting that claim.
    – Carey Gregory
    Jun 10, 2016 at 21:22
  • @CareyGregory You shouldn't downvote him for that. He was explaining possible cause for what I claim. Jun 10, 2016 at 22:30
  • @TomášZato I believe his explanation is largely incorrect, and since no supporting sources were offered to support the claims he made, that buys a downvote. That's actually a very good reason to downvote.
    – Carey Gregory
    Jun 11, 2016 at 1:14
  • I downvoted because no references to the claims. The links are about general human biology. The second to last paragtaph should link to a reliable book or source or at least something. I can remove my downvote if its improved. Jan 12, 2017 at 17:54

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