I have had some symptoms lately of heat exhaustion after trainining rides...75 minute rides where my heart rate gets near 180 3 or 4 times during the ride. I really don't notice it that much until I get back home where I start to feel light headed and lethargic. I'm not really back to normal until the following afternoon. On longer weekend rides where the pace is not nearly as hard, I have no symptoms at all(and the weather is just as hot). If it is a hydration problem, I'm wondering if I should be drinking a sports drink during the day of these evening training rides...right now, I just hydrate myself with water. Could not enough sodium intake be a problem? Any ideas would be greatly appreciated.

Thanks,
Scott Hager