I have struggled with the mid-August heat for the past 3 years. In 2009 it was at its height when I was working in the garden in the morning trimming a tree, not wearing a hat because I thought that the morning heat couldn't possibly be dangerous (Norfolk, UK is not generally known for scorching temperatures, but is well known for being the driest region of Britain). I felt very physically and mentally tired after working for only about an hour. I went inside and had plenty to drink, but within another hour or so I was sitting in the living room with a blanket around me and with a headache. The headache became worse, and the feverish conditions I was experiencing became more emphasised. I struggled on for the next week or so, with conditions escalating more into me having an abscessed tooth and thus only being able to eat liquids. I was bed-ridden for about a week, and did not regain full health for about 2 weeks.
The same thing happened the following summer, at around the same time in August. This time, though, I was prepared for it. I stopped what I was doing at the time when I felt the symptoms coming on, and wrapped up straight away. I made myself plenty of hot drinks, and felt the need to eat lots of high-carb foods (potatoes were amazing). The symptoms did not escalate this time, and I felt myself become better within a few hours.
Last summer (2011) I had it come on again a little, and just made sure that I kept myself warm and full of food. Again, the symptoms disappeared fairly quickly.
It is important to note also that for the past two years in the summer I have been wearing a hat pretty much every day. I also wear sun glasses to protect my eyes against the glare. I don't like wearing sun tan lotion, so I just make sure I cover my neck because that's the only area that normally burns when I'm dressed.