Penn State University researchers claim we need eight and a half hours for sleep. Their reason? Most people only sleep 90% of their time in bed, so you need that extra half hour if you want to have eight hours real sleep.

Good sleep is worth the effort – but how long do you need?

0 hours Total sleep deprivation is literally torture. After three days you lose all sense of what is happening, and the body shuts the brain down in “microsleeps”. Eventually, you die. Not recommended.

Two hours No one lives with only two hours’ sleep every night. Yet many of us may end up with just two hours on special occasions. Not recommended, except in emergencies.

Four hours Some politicians and CEOs boast that they need only four hours’ sleep, but it simply isn’t true. Take Donald Trump, for example. If this were the case, the scientific evidence would suggest he would be overweight, show erratic behaviour, poor judgment and frequent mood swings. Arianna Huffington said she slept so little that she eventually collapsed. Since her recovery she makes sure she gets eight hours’ sleep. Four hours is not recommended, especially if you are in charge of a country or big business: dangerous for you – and the people around you.

Six hours A lot of people get only six hours’ sleep on workdays. “Thank God it’s Friday” has a scientific basis: your sleep loss has built up with each work night and you look forward to sleeping in. Working hours are the greatest cause of losing sleep. Ask for flexible hours or home-working and time office meetings for the afternoon.

Eight hours This is the widely recommended amount you should sleep each night. Body clocks vary of course, so while Bill Gates finds seven hours is what he needs, you might need a little more.

Ten hours Teenagers need to sleep longer – about nine hours a night. Early school starting times are probably the greatest cause of teenagers losing sleep. Moving starting times later – as late as 10am – has been shown to reduce illness and increase exam scores.