Glenn and his fellow travelers are unlikely to lose excessive amounts of bone after such a short flight, but some astronauts, such as those living for months on the Russian Mir space station, never regain strong bones. This makes returning to Earth dangerous, and is something we must take into account if ever attempting long-term space travel. These alanine molecules were tagged with a special marker allowing researchers to see where the alanine ended up. Similarly, to measure protein breakdown, the pair were injected with the amino acid histidine, also labeled with a marker molecule. Another problem faced by all astronauts is fatigue caused by sleep deprivation.
Most astronauts sleep for just six hours.
36 to 48 hours straight. This is obviously a dangerous situation. Why it happens, nobody can say for sure. 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.