Fridays are the worst day, closely followed by Mondays, according to a poll of 3,000 British workers.
Two-thirds of people feel over-tired after the weekend, with workers spending at least half an hour making coffee, reading the paper and chatting with colleagues.
Nearly half said they spent the first day of the week organising and planning their workload.
Four out of ten said they deliberately scheduled meetings for Tuesday – knowing that, by then, they would have a precise idea of what had to be achieved, e-mails would have been replied to and the events of the weekend would be firmly behind them.
Almost a quarter said they started thinking about the weekend on Wednesday, while a third said by Friday they were counting the minutes until they could leave the office.
But at 11:33am on Tuesdays, office workers said they were feeling awake, organised and on top of their game.
Cary Cooper, professor of organisational psychology and health at Lancaster University Management School, said the results showed the increasing amounts of stress in the workplace.
"People are not at their peak on Monday mornings," he said. "They are wound up and stressed, thinking of all the things they have to do. By Tuesday, people have got rid of a lot of their in-tray. People feel more relaxed, more patient and are likely to be more productive."
He went on: "Fridays are the days when everyone dumps on everyone else.
"Britain has the longest working hours in Europe. We work really hard and really long hours, and then when the weekend comes, we really need it."
Professor Cooper said smartphones and e-mails had led to increased stress in the workplace. "Things are very different today because technology means people are available 24/7," he said.
"What people really should do is relax and unwind at the weekend. But it is very difficult for employers to know what to do because we have let the genie out of the bottle."
The survey was carried out for toothpaste manufacturer Arm and Hammer.
A company spokesman said: "The majority of Britons clock in with Monday morning blues at the beginning of the week, and the combination of a heavy weekend and a heavy workload mean it is difficult to focus on anything.
"But by Tuesday, workers are at their highest performance-wise – managing to shift the majority of their tasks, shine in meetings and impress clients with their efficiency.
"Unfortunately, things start to dip again by Wednesday as the enthusiasm wears off slightly and people begin to focus on the social events of the following weekend.''