The science behind the numbers that govern our lives
Think about your average day as a series of choices. You'll get up, you'll choose what to eat, whether to go for a run, whether or not to indulge in a glass of wine or a second helping of dessert. You're constantly making decisions based on what you want versus what you think is good for you. And how do you know what's good for you? Because somebody told you so. On average, we are given 123 different pieces of advice every week, from sensible government guidelines to scare stories about what gives us cancer. It all proves the point that you can assign a number to just about anything. Here's the latest thinking about the science behind the numbers that govern our lives.
35 is the upper limit for women having children
"Have your babies before this clock strikes 12," Professor Mary Herbert, a specialist in reproductive biology, told an audience at the British Science Festival this year. "I would be getting worried about my daughter if she hadn't had a child by 35."
Herbert's Cinderella analogy may raise a few eyebrows, but she's certainly not the only expert keen to tell women who want to be mothers to get on with it. "I read one paper that referred to eggs as 'best used by 35'," says economist Professor Emily Oster.
"Thanks; it's really helpful to know my sell-by date."
Oster became interested in the "fertility cliff" when researching her book Expecting Better, which tackles the data behind the most common - and controversial - pregnancy advice.
She found that the main research on fertility rates comes from 19th century data, based on the age of women at the time of their marriage. The theory was that "couples would pretty much get down to business right after the wedding ... Researchers found the chance of having any children was very similar for women who got married at any age between 20 and 35. Then it began to decline: Women who got married between 35 and 39 were about 90 per cent as likely to have a child. Women who got married between 40 and 44 were only about 62 per cent as likely."
So, yes, there is a decline, but only a 10 per cent difference between the first two age groups, a century before in vitro fertilisation. Perhaps that clock isn't going to chime 12 quite as soon as you think.
8 glasses of water a day
The claim that we should drink eight glasses of water a day is widely attributed to a report from 1945, from the American National Academy of Sciences' Food and Nutrition Board, which estimated we needed one millilitre of water for every calorie of food.
"What it meant was that daily fluid turnover is 2.5 litres, which is the equivalent of eight glasses of water," sports scientist Timothy Noakes says. "The fluid does not need to be water. About 750ml comes from food eaten each day."
The eight glasses idea might seem fairly harmless, but it has fed into the belief that we should all be drinking more water, that it is healthy to be proactive about "staying ahead of our thirst". It's an idea now widely promoted in sport, but as Noakes details in his book Waterlogged: The Serious Problem of Overhydration in Endurance Sports, even athletes weren't really encouraged to take on more fluids while training until the late 1970s.
It was then that sports drinks first appeared and more funding was given to research that promoted the "science of hydration". There have been no reported deaths from dehydration in sport, but there have been several deaths caused by hyponatraemia, or water intoxication. These cases are rare, but they indicate how confused we've become about what our bodies need. If you're thirsty, have a drink. If not, you're probably fine.
2,000 calories a day for women, 2,500 for men
In the film Super Size Me, Morgan Spurlock's camera crew ask people on the street to define "calorie". Most can't. As Marion Nestle and Malden Nesheim recount in their book Why Calories Count: From Science to Politics, Spurlock's crew "could not find even one person who could come up with a reasonable definition".
There are actually five different measurements for calories as a unit of energy. The guideline we most commonly think of - 2,000 or 2,500 a day - is in calories with a small "c". Around two-thirds of the total calories you need are defined by your basal metabolic rate (BMR) - the amount of energy you expend just existing: that's breathing, brain function, blood circulation. On top of that, anything you do, including shivering or fidgeting, will increase the number of calories you need.
Your BMR is affected by your weight and height (different formulas will give different totals), and the amount of activity we do each day varies greatly. So do the average calorie guidelines really apply to any of us?
"It's hard to arrive at anything approaching the correct number without doing expensive tests involving non-radioactive isotopes," admits Nestle. So, would we be better off using one of those online calculators that gives us an individual number? "Good grief, no," she says. "If you are going to buy anything, get a scale that works. And use it."
8 hours' sleep a night
For every person who will tell you that Margaret Thatcher got by on four hours a night, a study will show that the average person needs between seven and nine hours of sleep to function well. If you regularly average less than seven hours, you raise your risk of depression, diabetes and heart problems. But sleeping for more than nine hours a night has been associated with an increase in the likelihood of physical and mental-health issues.
Myths about our sleep abound. Is every hour of sleep before midnight worth more than the hours after? There's little evidence to support this, although one study showed that people performed better when they'd had more sleep after midnight, contradicting the myth. Do you need to get your sleep in one block? Again, this may not be true. Some historians believe that "segmented sleep" used to be the norm.
Roger Ekirch made a study of sleep throughout the ages in his book At Day's Close, and found many references in different languages to a "first" and "second" sleep. People would get up between the sleeps, and use the time to pray, reflect on their dreams, have sex or even visit neighbours.
Dr Thomas Wehr of the US Institute of Mental Health asked patients to spend a month living without artificial light. By the end of his experiment, the patients had all fallen into a pattern of sleeping for a few hours, waking for an hour or so then sleeping for a few more. They reported never having felt so rested.
30 minutes' exercise, 5 times a week
You've probably heard that the recommended amount of exercise is half an hour's moderate aerobic activity at least five times a week. The official guideline from the British Department of Health is at least 150 minutes a week, so it suggests you break it up into five half-hour sessions.
But, within that, there are other suggestions. On two days or more a week, your physical activity should include strengthening exercises that work all the major muscle groups. Oh, and instead of 150 minutes of moderate exercise, you could do 75 minutes of vigorous intensity activity, "such as running or a game of singles tennis". Or, if you like, you can do a mix of the two. Not exactly catchy, is it?
2 hours a day of screen time for children
Given the amount of panic there is about children watching TV, playing computer games or going online, there is surprisingly little research into the long-term effects of screen time. That's not to say studies don't exist, but they are scattered and the quality of the sample sizes and data gathered varies. This is also an area where the research we do have is likely to be sensationalised - such as the claim, in 2009, by Dr Aric Sigman that the isolation caused by social networking sites could give you cancer.
It's widely accepted that being sedentary for too long leads to physical health problems - although no one worries whether children are reading for more than two hours a day. So the more difficult question is: should we limit screen time to protect our children's emotional health?
In August, Public Health England, a government body, published a briefing that analysed a number of datasets on children's well-being. It included research from the Millennium Cohort Study, which collected reports from the mothers of 11,000 children at age five, and then again at seven, making it one of the largest data samples available.
That study found that watching TV for three hours or more at age five predicted a slight increase in behaviour problems at seven, but there was no link between playing computer games and behaviour problems, and "no associations were found between either type of screen time and emotional symptoms, hyperactivity/inattention, peer relationship problems or pro-social behaviour".
So, instead of worrying that our children's attention spans are shrinking, should we marvel at the way the new generation of digital natives effortlessly parse information on their phones and start online uprisings that change the world? It's probably too early to tell.