In some jobs, employees regularly work outside of the normal office hours of 9am to 6pm, as their co-workers or clients operate in a different time zone on the other side of the world.
Such work hours, in the long run, can disrupt employees' biological clock.
The good news, however, is that by taking small practical steps, one can enjoy his work and keep his health intact, too.
"If you cannot avoid shift work, there are ways to reduce its health risks," said Dr Fong Yuke Tien, senior consultant and director of occupational medicine at the Singapore General Hospital's Department of Internal Medicine.
The human body's circadian rhythm can be thought of as an internal 24-hour biological clock that regulates and synchronises the body's physiological processes, according to the natural patterns of daylight and darkness.
This internal clock instructs the brain to release more melatonin - a sleep-inducing hormone - when it detects less sunlight in the external environment.
This is why most people feel sleepy at night.
"Shift workers work odd hours and under artificial indoor lighting," said Dr Fong.
"During a night shift, the body will adapt to a changed sleepwake pattern. When this pattern of wakefulness at night is persistent over weeks, hormonal adjustments will occur to allow us to adapt to staying awake at night and sleeping in the day," he said.
Those who work odd hours and experience headaches, insomnia, excessive sleepiness and poor concentration could be suffering from shift-work sleep disorder.