The pilot was conducting a flight to satisfy distance, height and duration requirements for the silver endorsement on his gliding certificate. One of the requirements was that the flight should last for at least five hours. After about three hours flying the pilot felt slightly nauseous, and about an hour later he became extremely nauseous and vomitted. His condition then improved, and he elected to continue the flight. He subsequently arrived in the circuit area at an indicated altitude of 2500 feet, and intended over- flying the field before joining a left circuit to land. He was aware that the angle was unusual but deferred to his altimeter, assuming his perception had changed as he had been above 5000 feet for most of the flight. He then realised that the aircraft was much lower than the indicated height and that a landing on the intended strip would not be possible. While manoeuvring to land on another strip, the left wing tip touched the ground and the aircraft swung through about 120 degrees before striking the ground heavily. After the accident the pilot discovered that he had inadvertently set 1000 feet instead of zero on his altimeter before DEPARTURE. The pilot was aware of the problems of sun exposure and had taken suitable precautions. These precautions were apparently insufficient to counter the effects of dehydration and heat stress.