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Most of us know that mid-afternoon slump when you just can’t find the energy to write a long reply to a newly arrived email.

In that moment a curt “no” seems to sum it up perfectly. It’s not that you’ve suddenly turned rude – it’s all because it’s the wrong time of day.

Researchers from the University of Southern California and Yahoo Labs analysed 16 billion messages in the largest ever study of email communications.

And one of their main findings is that messages sent in the morning get much longer replies.

So exactly what else is it best or worst to do before or after noon?

Here’s our guide...

Smoking

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OK, it’s never going to be good for you but the worst time to have a puff is early in the morning, because the sooner you have a cigarette after waking up, the more likely you are to get lung and mouth cancers, no matter how much you smoke each day.

Researchers at The Pennsylvania State University found first-thing smokers have higher levels of NNAL, a substance produced when the tobacco-specific carcinogen NNK enters the body.

Go under the knife

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The best time to have an operation is between 9am and noon. according to a US study of 90,000 procedures, “adverse events” were least common in the mornings and most likely to happen between 3pm and 4pm, so avoid these times if you can.

Mornings are also a good time to have injections because our hormones that try to neutralise pain are at their peak efficiency.

Drink coffee

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First thing in the morning is the worst time to have a cup because you’re unlikely to feel the benefits.

Our cortisol levels are at their peak from 8am-9am and drinking coffee at that time can result in a higher tolerance of caffeine.

Which means you’ll be asking for an extra shot in your morning mug to get the same effect.

Steven Miller, a neuroscientist in Bethesda, Maryland, says the best time to grab a coffee is actually between 9.30-11.30am, after that cortisol peak.

Making your mind up

At 11.59am the morning rush is over and for most of us the lunch hunger pangs have yet to kick in and distract you.

So this the best time of day to make an important decision according to a recent survey carried out for the UK ticket website thetrainline.com.

And the worst time to make a life-changing decision is 10pm. Our brains are frazzled after a hard day’s work and our bodies are ready for bed.

Have an argument

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You may think it would be best to get things sorted before getting on with your day.

But you’d be wrong. Early mornings are the worst time to have a difficult conversation or argument.

The reason is that 6am-noon is the peak time for cardiac arrests.

All that extra stress might just prove fatal.

Have sex

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At 8am, your energy and hormone levels are at their peak. And if you’re trying for a baby, male sperm counts are higher too. But don’t overdo it.

One US study says the optimal duration of sex is 7-13 minutes.

And experts at Brunel University back this up, warning that too strenuous a session at this time of day could suppress your immune system.

Have a filling or a facial

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The perfect time of day to book an appointment at the beauty salon is 2pm, because moisture levels in your skin are at their highest.

Not only that, it’s also a good time to visit the dentist. local anaesthetic lasts longer when given at this time.

Go on a date

Research shows you’re at your most happy, relaxed and sociable at 7pm, so that’s the perfect time for a date.

And if that includes drinking, your liver will be working at optimal level from 7-8pm, meaning you process the alcohol most effectively.

Creative thinking

Problems that need open-ended thinking are best left until the evening.

A study of more than 400 students in Michigan found that the more tired we are, the more our mind wanders and so the more creative we become.

Post on social media

Social media scientist Dan Zarrella says most of us lack the energy to post our own tweets in the afternoon so we share other people’s instead.

So if you’re after a boost in popularity, the best time to post your witty words is from 3-6pm.

Facebook posts willl get more “likes” at around 8pm because people have usually eaten and stress levels from work have died down.

Exercise

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At 5pm, our lungs are nearly 20% more powerful than at noon, says Dr Boris Medarov, an assistant professor of medicine at Albany Medical College in ew York.

Muscle strength tends to be at its highest between 2pm and 6pm so that’s the best time to pick up a club or racquet and our hand-to-eye co-ordination is also best in late afternoon.

Get a job

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The worst time for a job interview is 4pm-6pm, according to professors at the University of Pennsylvania’s Wharton School and Harvard Business School.

They spent 10 years analysing data from 9,000 MBA interviews and found that when five similarly qualified candidates were seen on the same day, the last person to be interviewed was scored lower than they should have been.

This happens because if you are interviewed after several good candidates the interviewer will subconsciously bet on “bad”, much like gamblers playing roulette after a string of favourable results.