``I feel young all the time... I'm having a great time,'' Glenn told a group of students in a two-way radio link between the space shuttle and Glenn's former high school in New Concord, Ohio.

The students, whose school was renamed `John Glenn High School', posed a range of questions to the 77-year-old US senator, asking him about himself and the experiments on ageing that are being conducted aboard Discovery.

Glenn said that things in space had changed since his Friendship 7 rocket took off in 1962 for three quick orbits around Earth. ``I think I was a lot more nervous back in these days,'' he said.

The seven-member space crew was awakened on Saturday to the strains of Nat King Cole's Cachito, a Spanish song about the good fortune of being a parent, dedicated to Spaniard PedroDuque, a first-time space traveller who recently became a father.

Shortly afterwards, Glenn began his work as a human guinea pig undergoing tests to compare the physiological effects of weightlessness on ageing and, more specifically, on astronauts.

It was the US senator's first trip into space since he became the first American to orbit Earth in 1962. With this trip, he becomes the oldest man to travel in space.

After ingesting special amino acids to allow the detection of protein changes in their blood, Glenn and Duque gave blood and urine samples. The tests are expected to help find a treatment for use on Earth-bound humans who have suffered muscle loss and osteoporosis since both problems are exacerbated by the near weightlessness of space.