What difficulties did the astronauts encounter in sleeping during the four-day Gemini mission?

The four-day Gemini mission provided the first opportunity to seriously evaluate space sleep. During this flight, astronauts encountered great difficulties in obtaining satisfactory long-term sleep. No astronaut can sleep for a long time. They can only sleep for four hours at most. The commander himself estimated that he didn't get more than 7.5 to 8 hours of good sleep during the whole four-day flight.