“Staying up all night is my habit,” Kim was quoted as saying by the North’s Rodong Sinmun newspaper in a June 22 article about his lifestyle, South Korea’s Yonhap news agency reported on Sunday.

Kim, 65, also appears to make phone calls to officials during pre-dawn hours to give instructions. In one such pre-dawn phone call to a senior provincial official, the unidentified official asked Kim to take a rest, even if just a brief one.

“General, how could you stay up all night again?” the official said in the 4:20 a.m. call, cited by Rodong Sinmun, the official paper of North Korea’s ruling Workers’ Party. “For me, this is the prime time to work,” Kim allegedly replied.

Lee Young Kook, who served as one of Kim’s elite bodyguards for a decade before defecting to South Korea in 2000, told AP in response to the report that Kim usually stayed up overnight to sign documents about 20 days a month. Kim “dropped by his office after midnight and left there around 11:00 a.m.,” Lee said.

Kim’s health has came under the media spotlight amid unconfirmed news reports that he received some kind of medical procedure involving his heart in May, performed by doctors flown in from Germany.

The German Heart Institute Berlin confirmed it sent a team of doctors to North Korea in May, but said they performed operations on five people not on Kim.

Adding to speculation regarding Kim’s health was a rare public appearance last week during a meeting with Chinese Foreign Minister Yang Jiechi in Pyongyang.

Though Kim looked well, he seemed a bit thinner and appeared to have less hair compared with footage of a public appearance in April, sparking speculation in South Korean media about his health. The South’s spy agency, the National Intelligence Service, said last month that Kim has long had heart disease and diabetes, but that there was no sign the chronic ailments.