Drinking, smoking, being overweight and taking drugs have little impact on a key aspect of the sperm quality of infertile men, according to new research.
Men with unhealthy lifestyles produced as much swimming sperm as those living more sensibly.
The surprise findings contradict advice given to men who struggle to conceive.
Under current guidelines, GPs are supposed to warn men diagnosed with infertility of the dangers of alcohol, tobacco and recreational drugs.
Infertile men are also urged to avoid being overweight and not to wear tight underwear.
In some cases, fertility treatment is delayed to allow couples time to improve their lifestyles.
But the new research suggests none of these factors has much impact on the number of swimming sperm a man produces.
Scientists recruited 2,249 men from 14 fertility clinics around the UK and asked them to fill out detailed lifestyle questionnaires.
Information was then compared from 939 men who produced low numbers of swimming sperm and 1,310 men who produced higher numbers.
The results, reported in the journal Human Reproduction, showed that men with poor quality sperm were 2.5 times more likely to have had testicular surgery, and twice as likely to be of black ethnicity.
They were also 1.3 times more likely to do manual work, not wear loose boxer shorts, or not to have had a previous conception.
But men's use of tobacco, alcohol and recreational drugs made little difference, as did their weight as measured by body mass index (BMI).
Study leader Dr Andrew Povey, from the University of Manchester, said: "Despite lifestyle choices being important for other aspects of our health, our results suggest that many lifestyle choices probably have little influence on how many swimming sperm they ejaculate. For example, whether the man was a current smoker or not was of little importance. The proportion of men who had low numbers of swimming sperm was similar whether they had never been a smoker or a smoker who was currently smoking more than 20 cigarettes a day.
"Similarly, there was little evidence of any risk associated with alcohol consumption.
"This potentially overturns much of the current advice given to men about how they might improve their fertility and suggests that many common lifestyle risks may not be as important as we previously thought.
"Delaying fertility treatment then for these couples so that they can make changes to their lifestyles, for which there is little evidence of effectiveness, is unlikely to improve their chances of a conception and, indeed, might be prejudicial for couples with little time left to lose."
The number of swimming sperm broadly correlates with how fertile a man is likely to be. It also often determines the type of fertility treatment that is offered.
Co-author Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said men should still take care of themselves, without feeling the need to "become monks".
"Although we failed to find any association between common lifestyle factors and the number of swimming sperm men ejaculate, it remains possible that they could correlate with other aspects of sperm that we have not measured," he said. "These include the size and shape of sperm (sperm morphology) or the quality of the DNA contained in the sperm head. We need to do further work to look into these aspects.
"In spite of our results, it's important that men continue to follow sensible health advice and watch their weight, stop smoking and drink alcohol within sensible limits. But there is no need for them to become monks just because they want to be a dad. Although if they are a fan of tight Y-fronts, then switching underpants to something a bit looser for a few months might be a good idea."
Fellow expert Professor Nicola Cherry, formerly at the University of Manchester and now with the University of Alberta in Canada, added: "The higher risk we found in manual workers is consistent with earlier findings that chemicals at work could affect sperm and that men should continue to keep work exposures as low as possible."