BBC's Tom Carver: John Glenn has spent the time acting as a guinea pig

They have seven and a half hours of free time to relax and take in the view from space.

The crew are also catching up with loved ones back on Earth, holding private 15-minute video conferences with family members.

It is their first break from an intense four days in space, following Thursday's launch from Cape Canaveral.

The mid-mission break is scheduled into all shuttle flights to prevent crew fatigue.

Spartan: Mission to study the sun

The most famous name on the shuttle, Glenn, is being treated like another of the crew and not getting any extra time off because of his age.

On Sunday night, he donned a helmet packed with 23 sensors which recorded his sleep patterns.

He and Japanese astronaut Chiaki Mukai are wearing the helmet for four nights as part of the research on ageing that is his primary task during this nine-day mission.

James Helm: Glenn encourages Americans to vote in mid term elections

Glenn's role on the flight has been criticised as a publicity stunt by Nasa to drum up interest in the space agency.

The astronauts are carrying out more than 80 experiments. On Sunday, they released the $9 million Spartan solar physics laboratory equipped with instruments to study the sun's corona and the solar winds.

They are scheduled to retrieve the satellite on Tuesday after two days of flying free from the shuttle.