IT HAS been a case of numerous 'hard Day's nights' for Aussie Jason Day in the lead-up to this week's US Open at the imposing Chambers Bay layout in Washington State.
The world No.10, who had to withdraw from last month's Byron Nelson Classic after experiencing severe dizziness, has also been having sleep issues.
But after the results of numerous tests - including three sleep studies, blood tests and MRI scans on his head, neck and back - came back negative, Day declared himself "good to go".
"I've got severe sleep deprivation, so I guess that's part and parcel of having a kid," the Queenslander said. "That's just life, so you've just got to deal with it.
"I feel good this week."
Day, who played a practice round at the beachside course with Tiger Woods and Dustin Johnson on Monday, attributed the mysterious illness to the exhaustion of working out more, changing his diet and trying to balance a young family.
Still chasing his first win in a major championship, Day will tee off alongside Masters champion Jordan Spieth and Englishman Justin Rose in the first round.
Other Aussies in the field are Adam Scott, Marcus Fraser, Cameron Smith, Kurt Barnes, John Senden, Geoff Ogilvy and Marc Leishman.