I did an experiment today to see the effect of bright Sunlight on post meal blood sugar. This is the first time I did this exercise.
With my usual Whole plant based diet , I used to get my fasting blood Sugar @ 87-102 mg/dl and PP of 108-115 mg.
Today My fasting blood sugar was 97mg/dl. I had my usual breakfast @9 AM and thereafter my usual 30 mnts Post meal treadmill walk.
Then , did my usual household chores. At about 10.30 AM- it was sunny , hot temp @39 degree C, went to my terrace and exposed my body in bright sunlight, sitting in a chair for 20 mnts. Then at about 11 AM , I checked my PP sugar and found to be 124 as against my usual reading of 108-115 PP.
Then I was trying to understand , what made my blood glucose to spike?
I came across few articles which says that some studies also indicate similar results for which possible reasons are :
(1) Hot Sunlight can alter your metabolism
(2) Hot sunlight could induce physical stress which could make your liver to release sugar to the blood stream .
(3) Dehydration could also be a reason. .But I took enought water post meal before I went for sunlight exposure , though I was sweating a bit .
Anyway, this is important to note and you may also try to do the same exercise and post your result. If you want to do this exercise , please donot add any other variables. Don't stand / sit in cloudy weather or cool sunlight. . You should be able to sweat a bit...In any case, every body is different and you may or may not react to bright sunlight.
This is an important point applicable for me because I need to be careful though sunlight exposure has helped me maintain adequate vitamin D level and I had stopped D- supplements . I normally used to go for sun bathing 3 times a week @11.30 AM..though I had never done this exercise before.
My Fasting blood sugar level increased after work out..
Fasting Glucose is more than PP
Sugar Level Increase Before Breakfast 
illness and blood sugar rising
