O'Sullivan wrapped up his sixth UK Championship title on Sunday and looked to be suffering the effects as he laboured past Li Hang on Thursday afternoon, during which he could be seen closing his eyes when not at the table.
He told Eurosport: "My doctor's told me to catch some rest whenever I can because he knows I've had a busy eight weeks.
"So even when I thought I shouldn't be sleeping, I've just been sleeping and it's working.
"It's just a little snooze - everyone needs a snooze."
Returning for the evening session, O'Sullivan appeared refreshed as he saw off Michael White 4-1, the second tournament in succession in which he'd ended the hopes of his opponent, who had earlier beaten Kyren Wilson.
O'Sullivan's victory sets up a mouthwatering quarter-final clash with John Higgins, who won all eight frames he played on Thursday.
First, the Wishaw man beat Gerard Greene, before taking time out to watch his daughter in her school's Christmas Nativity play.
Higgins returned to the arena to end the hopes of 23-year-old Ashley Hugill in a one-sided clash.
Higgins' fellow Scottish hopeful Stephen Maguire had to battle to beat Yan Bingtao 4-3 earlier in the day, but was on top form later, beating Xu Si 4-1.
He'll face Judd Trump for a place in the semi-finals after the Bristolian overcame Elliot Slessor 4-3 in a tight tussle which ended shortly before midnight.
Neil Robertson signed off with back-to-back centuries to beat David Grace and secure a quarter-final spot, having earlier thumped Chris Totten 4-0.
Next for Robertson is a clash with Xiao Guodong, who knocked out defending champion Marcu Fu in a thriller to kickstart the evening session.
Xiao had led 3-0 only to be pegged back at 3-3, before a brave 73 break in the decider saw him advance.
Cao Yupeng beat Tom Ford 4-0 and will face Ricky Walden, who edged past Rory McLeod 4-3.
McLeod had beaten Ding Junhui earlier in the day but this time lost the deciding frame to Walden, who had been reeled in having led 3-1 but found an important half-century when it mattered most.
Third round (Best of 7 frames)
Morning session
- Marco Fu 4-3 Zhou Yuelong
- Noppon Saengkham 1-4 Xiao Guodong
- David Grace 4-3 Zhang Anda
- Chris Totten 0-4 Neil Robertson
- Tom Ford 4-0 Stuart Carrington
- Cao Yupeng 4-3 Daniel Wells
- Ashley Hugill 4-1 Peter Lines
- Xu Si 4-2 Craig Steadman
Afternoon session
- Michael White 4-1 Kyren Wilson
- Li Hang 3-4 Ronnie O'Sullivan
- Ding Junhui 3-4 Rory McLeod
- Mark Joyce 1-4 Ricky Walden
- Yan Bingtao 3-4 Stephen Maguire
- Elliot Slessor 4-3 Peter Ebdon
- John Higgins 4-0 Gerard Greene
- Michael Holt 0-4 Judd Trump
Fourth round (Best of 7 frames)
Evening session
- Marco Fu 3-4 Xiao Guodong
- Neil Robertson 4-2 David Grace
- John Higgins 4-0 Ashley Hugill
- Michael White 1-4 Ronnie O'Sullivan
- Rory McLeod 3-4 Ricky Walden
- Tom Ford 0-4 Cao Yupeng
- Stephen Maguire 4-1 Xu Si
- Elliot Slessor 3-4 Judd Trump
Related links
Fast Results
Scores
Racecards 
