I am a Texan, and work outside in the yard lots - some work easy, some hard - and in the all day heat (though today was actually a "good" day at my house, never hit 100 degrees!). Anyway, I find with insulin and heat - day one and two are no trouble, insulin works with no trouble and I do not need to adjust amount bolused for carbs. Usually into day 3 and for sure if I do not change sites prior to day 4, I find a tendency for unexplained highs and I must increase insulin amount to cover carbs and usually my basal rates must also increase. Once I do a site change, all is back to normal. In the cooler months (winter), I find that if I go into day 4 before a site change I am usually fine. Therefore I blame the summer changes in the length of time my insulin is "good" on the heat - but the changes in sugars worsen as the day progresses and are not dependent on carb intake or time of day.