"To be honest I haven't been sleeping well recently," the Celtic midfielder told Thursday's Japanese media.

"I can't sleep for more than three hours so I've been trying to catch up by taking daytime naps. My body's not in the best shape."

Nakamura was instrumental in Monday's 3-0 win over Oman in Yokohama and Japan must take three points in Muscat to keep the pressure on Group Two leaders Bahrain.

Japan's players were still trying to acclimatise to the Middle Eastern heat after chilly temperatures at home.

"I've been living in a cold country (Scotland) for three years so my body is surprised by the heat," said Nakamura. "Your muscles ache if you play when you're dehydrating so it will be important to take on enough fluids."