How do astronauts experience night and day in space?

Astronauts experience day and night in the same way as people do on Earth. The only difference is that they do it 16 times in 24 hrs and we do it only one time. Open the covers over the windows and the light can be so blinding that astronauts reach for their sunglasses. read more

At Launch Pad 39A at NASA's Kennedy Space Center in Florida, STS-127 crew members gather near space shuttle Endeavour's hatch to place the mission plaque before launch. Clockwise from left are pilot Doug Hurley, mission specialists Julie Payette, Christopher Cassidy, Dave Wolf, Tom Marshburn and Tim Kopra and commander Mark Polansky. read more

So the idea that astronauts who "experience nocturnal emissions" would not be chosen is unlikely. These are not normally frequent. Most men do not experience nocturnal emissions except from relatively long abstinence from sexual activity (including masturbation). So they wouldn't know until they are abstinent for a substantial time. read more