It isn’t the sun as much as it’s the heat generated by it. Your body burns calories to maintain your body temperature, so in essence, you’re burning fuel. That tires you out.

The sun does not directly affect your stamina. It is the humidity, air temperature and the ability of your body to thermoregulate and compensate for these factors.

If your perspiration increases as a result of the humidity and air temperature, then you will become dehydrated, your stamina will decrease, and you will not be able to run as far. Thermoregulation also requires a degree of energy, so you will want to eat properly before you go out.

In addition, the heat can have a purely psychological impact, causing you to grow tired more quickly. Besides from practice, not much can cope with that.

Just take a large bottle of water with you when you go running on hot days and you should be fine.

Note: Just don’t spend too much time in the heat, as you may begin to suffer from hyperthermia (the opposite of hypothermia, and also more commonly known as ‘sun stroke’).

Hope this helps.

Thank you guys for the answers. So if I wanted to get the most out of my exercise I should run when it’s not as hot outside, because (correct me if I’m wrong) the longer you are able to run, the longer the amount of time your heartbeat will be increased, which in turn burns more calories. It would almost be a waste if I ran in the hot heat.

@RedmannX5: I’m not confident that it would actually make that much of a difference… not to mention running on a sunny day gives you a nice tan. :) If you would really like to find out, then try running on hot days and set yourself a marker. See how you feel at the same point on a cold day.

@Richard Henry: that’s good advice, I’ll have to give that a try. (and yes I have noticed some nice bronzing going on :) Thanks

Np, enjoy your run.

besides, the point of running is to get in shape and burn calories right? it makes no difference if you run in the sun or not. you get the same work out if you run till you are exhausted. Also, when I use to run track in highschool, practice was always in the middle of the day and that didn’t seem to affect any of us long distance runners times. Also, If you test it, run in the morning one day, and run in the middle of the day at like 2, you will probably not notice a huge difference in your mile, or what ever you run. Maybe if you live some where like arizona where its 120 you will, but obviously that is an extreme case, but temperature varying from 60 degrees to 98 degrees, not really that big of a deal.