“Those of us who like to sleep longer are considered lazy,” she says. “But it’s not true.”

* The amount of sleep we need is partly genetically pre-determined.

Traditionally, Western society has deemed that eight hours’ sleep is the right prescription for everyone. But it’s not as simple as that - and it no longer seems to be true.

A study of American sleep patterns in the 1970s found eight hours was the average, says Wirz-Justice. But 20 years later, the same study found the average had dropped to seven. It’s a phenomenon she expects is common elsewhere, including New Zealand.

So how much is enough? “The answer is a very individual one,” she says. Genetic and physiological factors come into the equation. Some of us, it seems, are destined to be long sleepers, others short. “There’s no judgment about what is better.”

Different stages of life determine how much sleep is needed too. Newborn babies spend most of their day asleep. “But as we grow older, [sleep duration] depends on the biological clock as well as the time of year,” says Wirz-Justice. “One important point is women would sleep longer than men had they the chance to do so - but they never do.”

Another important factor is the time of going to bed. Shift-workers, for instance, who sleep during the day would struggle to slumber more than four or five hours because their core body temperature is on the rise. “The body clock is sending out signals of wakefulness.”

In a 1991 study participants endured 14 hours of darkness - replicating the winter-time experience of our forebears, before artificial light - during which time they were not allowed to do anything but rest. The study found that the average sleeping duration was about 8 1/4 hours. Interestingly, however, the sleep was very disturbed, with long periods where they could not sleep. The participants, however, didn’t seem to fret, and woke feeling refreshed. Sleep did not have to be in a solid block to be worthwhile.

“So when you wake up in the middle of the night and can’t get to sleep, just lie there and think, ‘I’m in a medieval castle and everything is fine’,” says Wirz-Justice.

* Teenagers’ body clocks advance, making it harder go to sleep early.

Just as the duration of sleep is pre-determined, so too, it seems, the best time to go to sleep is influenced by individual factors. The world is divided into larks (early to bed, early to rise) and owls (late to bed, late to rise), says Wirz-Justice.

Some genetic factors are at play. Adolescence has an influence too. Between the ages of 10 and 20, the body clock adjusts by 15 minutes a year, before starting to slowly move back so that, by old age, people typically rise early.

“So it’s not that teenagers are going out and staying up, it’s that they can’t go to sleep,” she says.

She rails against the school day starting too early - in Switzerland, classes begin at 7.30am.

When we go to sleep is down to the interaction of two processes. One, called the circadian process, is the body clock, the internal mechanism influencing when we should sleep and wake. The other is the homeostatic process, governed by the amount of time we’ve been awake. The longer since we have slept, the greater the pressure there is to sleep. The body builds up a substance which increases the pressure to doze. What that substance is remains a mystery - so far.

* Grandma was right: wearing bed socks does help you get to sleep.

One of her most widely reported discoveries was the research published in 1999 that showed increased blood flow and heat loss through the body’s extremities was an important part of falling asleep. Several things happen when we nod off at night: in the evening, the so-called darkness hormone melatonin starts to rise; around the same time, blood flow and heat loss increases, aiding a drop in the body’s core temperature.

Warming your feet and hands, thereby dilating the blood vessels, helps aid the process. In a city like Basel, home to several pharmaceutical companies, it was not exactly popular to say that the best sleep may be brought about through a hot water bottle - or bed socks (which should be removed before falling asleep).

Activity during the day also aids sleep, although research has found it is not physical movement that is important so much as brain action. “You can do a marathon and sleep not much more,” says Wirz-Justice. “It’s the brain that’s important.”

* Eight hours’ sleep saves 120 calories - the equivalent of two peanuts

Sleep is a time for the body to rest, although the energy saving is literally peanuts and the metabolism only reduces by about 10 per cent. Studies suggest sleep may be about reprocessing our memories, getting our brains in order.

The two distinct stages of sleep each have a part to play. Early in our sleep pattern, we fall into a deep sleep, characterised by slow brain-wave activity, before moving into rapid-eye-movement (REM) sleep, during which the brain is active and dream-filled.

Both stages are important. Wirz-Justice says the deep sleep appears to be necessary to assist us with declarative learning - such as studying or learning a foreign language - while REM sleep appears necessary for procedural learning - such as playing an instrument or sport.

How and why that happens are still to be properly understood.

* Even a week of less sleep can start to have serious effects.

We may not know much about sleep, but we do know that the consequences of not enough can be extremely dangerous. Wirz-Justice bemoans the modern attitude to sleep, where businesspeople treat early mornings and late nights as badges of honour: sleep is the enemy standing in the way of progress.

The fact is, she says, the short-term effects of sleep deprivation are diminished executive function and decision-making. “Even a week of six-hour nights, instead of seven hours, shows substantially reduced alertness.” More mistakes and accidents are likely, learning is harder and the immune system is compromised.

The long-term effects are changes to glucose tolerance and insulin resistance, increased heart problems, and high blood pressure. In other words, sleep deprivation may be linked to another of society’s ills: obesity.

Take sleep lightly at your peril. Better to take Wirz-Justice’s advice: as you relax tonight and start to feel tired, have a warm cup of Milo, and slip on a pair of bedsocks - your body will thank you for it in the morning.