The average sickness absence rate stood at 0.78% after England's opening win over Croatia and fell to 0.75% following the draw with Ghana.
Robert Walters found that 35% of professionals expected to alter their working arrangements after the match, which finished at almost 4am.
The union body said businesses should, where possible, accommodate requests such as later start times, home working or adjusted hours.
18% of workers expect to spend between £100 and £250 during the tournament, while 14% plan to spend between £50 and £100.
The CIPD said organisations should think about how they can balance employee flexibility with operational requirements.
Employees and managers said they plan to skip work, turn up tired or hungover, and watch matches on the job.
Research from YouGov found that among Britons planning to follow the tournament, 26% said they would stay up past 11pm to watch matches live.