Elon Musk is tired, his back hurts and his mom wants him to get some sleep.

"I had trouble sleeping last night, so unfortunately, I'm not at my best," the billionaire entrepreneur recently told a lawyer during a trial in San Francisco. Later, Mr. Musk added: "I'm sorry for squirming around. I have quite severe back pain."  

A self-described nanomanager, Mr. Musk has long waded deeply into the weeds of the companies he runs, including SpaceX and Tesla Inc., TSLA 2.72%increase; green up pointing triangle routinely working late into the night and sleeping little. His tenacity has led to superhuman-like accomplishments, such as landing space rockets and making electric cars sexy.