B737-500 flight crewmember addressed poor scheduling practices for reserve pilots. Felt that allowing the schedulers to 'reset' reserves' rest period in the middle of a previously 'set' rest period results in the inability to obtain adequate rest prior to the 'now legal' later assignment.

Narrative

On the afternoon of occurrence; I received a call from the crew desk placing me on an XA00 short call for the next day. Due to the upcoming change to Daylight Savings Time; this was really the equivalent of the usual XB00 short call. The XB00 short call reserve block; exists for the purpose of having a reserve crew member available to cover an XF00 departure. Knowing that I was expected to be ready and rested to cover an XF00 departure; I went to bed early and did my best to get 7 or 8 hours of sleep in case the crew desk called and needed me. In fact; the crew desk did call me a little after XB00; but it was not to cover an XF00 departure. They called to say that they are resetting my rest period and now need me to cover a flight that night. This practice is something that the crew desk has started doing with some regularity; they've done it to me personally 2 or 3 times in the last month. While I'll concede that it complies with the letter of the law (crew rest requirements); it walks all over the intent. Not since I was a teenager could I sleep 16 hours straight. Having just finished 7 hours of sleep and a conversation with the crew desk; I could not possibly go back to sleep for another 8 hours. I had adjusted my body clock as best I could to an XB00 wake-up; and now was expected to fly till after XR00 that night. As is often the case; when I was on my way to the airport the crew desk called to say that my aircraft was delayed by 1 hour 40 minutes. It seems some weather in Chicago had slipped everything. I really felt pretty good through the start of the third leg. I am certainly willing to pull myself from a trip if I feel fatigued; but I felt like I was holding up pretty well. It wasn't until we were approaching Denver; well after XR00L; that it hit me like a sack of bricks. I was having problems concentrating; and could barely keep my eyes open. I had been doing everything in my power to stay awake (coffee; gum/candy; etc.); but as we entered one of the most critical phases of flight; I had been up for 20 straight hours. This situation was the direct result of an unacceptable scheduling practice. It is not enough to say that it was legal; or that I could have called in fatigued. Both of those are cop-outs. Numerous studies have shown that it is not possible to shift one's body clock by more than about 1 hour a day. The crew desk asked me to shift my body clock by 8 hours in the period of a 60-second phone call! Likewise saying that I could have called in fatigued sounds good on paper; but at FL370 and 30 minutes out from landing; it's not really much of an option. If we are really doing anything more than just paying lip service to safety; the practice of resetting a crew's rest period at the beginning of a duty period has to end. The crew desk needs to use early morning short call reserves to cover morning flying; and afternoon short call reserves to cover evening flying. Using an XB00 short call reserve to cover an entire day's flying is begging for trouble! It is certainly a clever loophole around the FAA's crew rest rules; but it clearly violates the intent of giving aircrews an opportunity to show up to work rested.