Astronaut wearing a sleep cap. US astronaut Richard M. Linnehan having a sleep cap fitted in the Neurolab onboard space shuttle Columbia during mission STS-90 (17th April - 3rd May 1998). Assisting him is US astronaut Jay C. Buckey Jr. (right). The cap monitors and measures electrical impulses from the brain, muscles, eyes and heart as part of a study of astronaut sleep patterns in space. The Neurolab is an onboard laboratory designed to test how human bodies adapt to microgravity conditions. Photographed on 21st April 1998.