Why Kim Soo Hyun sleeps only one hour a day?

"There's nothing he wants more than good night's sleep. Only had one hour sleep per day for a quiet a while now, We’ll have to pull all-night shoots tonight and tomorrow, too, to make the broadcast tomorrow.”

Officials at Kim Soo Hyun's agency gave update on his and "You who came from the stars" drama set's recent situation.
Filming takes place Monday through Sunday, and Kim Soo Hyun gets very little sleep. The representative said, “He had a CF lined up as well, so he filmed the CF and then went right back to the drama set at night. It is practically impossible to leave the drama set. Kim Soo Hyun plays an alien who came to earth 400 years ago, so he has to film scenes from the Joseon dynasty as well, so he has a really tight schedule.”

To present a high quality production to its viewers, the staff and actors are continuing their struggles day by day.

As the drama frequently goes from past to present time, there′s no avoiding more time put into the filming process, even with the extra time they earned with an recent episode cancellation due to Lunar New Year in Korea.

"Kim Soo Hyun is powering through the filming with around one to two hour naps a day. It′s been very hard for his body," said the agency staff. "But his determination to showcase his best acting skills and a great drama to his fans is great. We′ll repay with a good performance."

Not just sleep deprivation but words like "IV drip", "collapsed", "car accident", "fainted" and such are pretty common in the news about series and actors in Korea.
The reason why so many korean actor like Kim Soo Hyun is suffering from sleep deprivation during fliming drama is because of Korean drama's typical way of fliming, live-shoot system.
Very few dramas are completely filmed before they air (maybe about 6-8 episodes) to reduce cost of drama production, and that's why there is generally a quality dip around the middle of the K-drama. Many of the final episodes are written hours before filming with little time left for editing.

Biggest problem with the live-shoot system is that it's less about art and storytelling, and more about earning as much money as possible.
Despite the lack of sleep and last-minute script deliveries, the cast are able to keep going for the sake of ratings. Ratings bring along recognition, and recognition is directly associated with CF contracts and more CF contract means more money to broadcasting company and actors.
Kim Soo Hyun had about 10 CFs, but thanks to ratings approaching nearly 30% of his recent drama, he has added on banking, pharmaceutical and automobile commercials to his growing resume. For some, this could be a sufficiently sweet reward for lost sleep but fans of many actors and actress worry about their beloved star's health.