Factors such as the excitement of space travel, seeing a sunrise every 90 minutes and noise in the shuttle cabin all have their effects. NASA had planned to have Glenn take pills of melatonin, a hormone known to regulate the sleep-wake cycle. But this experiment was canceled shortly before takeoff, without explanation. Still, Glenn and Japanese astronaut Chiaki Mukai, the only woman on board, wore special vests while sleeping to monitor their breathing, heart rate and brain waves. Also, both swallowed capsules containing tiny thermometers and radio transmitters, which broadcast their bodies' core temperature to a recorder on their belts.
Many astronauts have trouble adjusting to zero gravity.
It is not uncommon for astronauts to spend their first days dizzy, nauseous and occasionally vomiting. Luckily, these symptoms usually subside, and the astronauts can function normally thereafter. But upon their return to Earth, they must once again reacclimate to gravity. Glenn needed a little assistance walking, but not much, considering that other astronauts have been wheeled off the shuttle in hospital gurneys. These balance problems affect everybody on Earth.