If your mind and body determined your working hours, you'd be in the office only four days a week for just five hours a day, according to two new studies.

Despite our culture's shift towards an "always on” world of 24/7 working, our hominid frames struggle to keep up - in fact they have trouble coping with the world of 9 to 5, says a new survey by Boots, the chemists.

British staff have enough energy to work only five hours a day, suggests the study. It claims that the average employee loses 151 minutes per day through tiredness. If we clock on at 9am, our brains take 45 minutes to kick into gear, it says.

Even if we take only half an hour for lunch, we fall into a postprandial slump at around 2pm, which can last for an hour and a quarter, the research adds.

You can pretty much forget about Monday morning and Friday afternoon and hope that the other days get the bulk of the week's work done, says another study. The survey of bosses, by the US recruitment firm Accountemps, found that 57 per cent said their staff were most productive on Tuesdays, compared with 12 per cent on Mondays.

Analysis shows that Monday mornings and Friday afternoons are indeed switch-off periods. So why bother rushing into work and staying there if we are so unproductive for much of the time?

Employers like to think they are getting value for the hours they pay for, but maybe we should all get more realistic. That idea is supported by another new study, by the UK employment law company, Peninsula, which reports that 63 per cent of workers have nodded off at work.

Such exhaustion significantly raises the risk of errors and accidents, says Peninsula's Deb Gibbon, who adds that one solution “would be to offer workers a short snooze break during the day”. Another might be to stop asking us to come into work so much.

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