Summer may be a time of joy for schoolkids and sun worshippers, but it's no picnic for employers. New research shows that there's a marked decrease in employee productivity during the summer, and that's not good for business.

According to new data in the Replicon Productivity Index, the slump begins the Tuesday after Memorial Day which, along with the Wednesday before Thanksgiving, is the most popular day for employees to take sick leave. It continues on through Labor Day: Employees are 52 percent more likely to use vacation time from June through August than any other time of the year -- even for spring break or Christmas.

"In today's economy, a decrease in employee productivity can pose a greater challenge to businesses than ever before," said Lakshmi Raj, co-founder and co-CEO of Replicon, which produces software that tracks time and attendance policies. "It can be difficult for businesses to strike a balance between accommodating employees' vacation requests and ensuring their productivity doesn't suffer."

But the summer slump is not the only interesting trend the Index shows in employee absenteeism. You'll probably be quite surprised about some of the other survey findings from the Replicon Productivity Index:

• Sick as a Top Dog: While it seems likely that employees would take off more sick days than managers due to having fewer vacation days, the survey found that managers actually take more sick days than non-managers per year.
• Weekend Hangover: Though many may assume that people kick off the weekend early by calling in sick on Friday, the Index found that Monday actually leads the five-day work week as the most popular sick day among employees. Friday and Thursday, respectively, rank as the least common sick days.
• Jack Frost Takes a Toll: Cold and flu season is to blame for much of the absences in the winter months (December through February). On average, 10 percent of employees call in sick during these months. This is 78 percent more than in summer months, when employees tend to take vacation time rather than sick leave.
• Holiday Blues: According to the Index, many people either kick off annual holiday vacations a day early or need an extra day to recover. The top three U.S. holidays that most often require an extra day of celebration or recovery include: Memorial Day, Labor Day and Thanksgiving.