The elderly do not need as much as the rest of us, adults wish they could get more of it and teenagers don't have any problems doing it all day. We are, of course, talking about sleep.
Only one in 10 of us believes we get enough sleep. But if we don't sleep as much now as we used to, it does not necessarily mean we are not sleeping enough. A recent study conducted at the Surrey Sleep Research Centre and published in medical journal Sleep earlier this month found that, of eight hours spent in bed, people in their 20s slept for an average of seven hours and 14 minutes. That fell to six hours and 50 minutes for those in their 40s and 50s, and fell again for the over 65s, who slept for an average of just six hours and 30 minutes.
Researchers believe that differences in sleep patterns are to do with changes in hormone levels or other changes in the brain.
Spokeswoman for the Sleep Council Jessica Alexander says that a function of sleep is to renew brain cells and it is reasonable to suggest that as a person ages, this happens less. She also feels it is to do with a change in health and lifestyle. She says: "When you get older, you potentially develop more health issues that can affect your sleep. Lifestyle habits also change in retirement."
Meanwhile, husbands should not complain if their wives insist on having a lie in, as women tend to need more sleep than men. This is because their brains are working harder. Naturally.
Professor Jim Horne, director of the Sleep Research Centre at Longborough University says: "One of the major functions of sleep is to allow the brain to recover and repair itself. The more of your brain you use during the day, the more of it that needs to recover and, consequently, the more sleep you need.
"Women tend to multi-task - they do lots at once and are flexible - and so they use more of their actual brain than men do. Because of that their sleep need is greater."
Do not get angry if you cannot drag your sulky teenager out of bed at the weekend - children and teens are another group who need more sleep. Their brains are a work in progress until the age of 21 and much of that work is done when they are asleep when blood to the muscles is increased, energy is released, tissue growth and repair occurs and hormones are released for growth and development.
But a recent survey carried out by the National Sleep Foundation in America found children are getting less sleep than experts recommend for their age group. Kathleen McGrath, a paediatric nurse and author of the Sleep Council's new Good-night Guide for Children booklet, says that attention, memory and learning are some of the first casualties of sleep shortage in children. It has even been linked to obesity and depression.
Mandy Gurney of Millpond Children's Sleep Clinic explains that this sleep deprivation is linked to modern life distractions such as TV, gaming and online social networking.
"We don't work with nature in the way that we used to," she says. "We have two main sleep triggers: change in light and a dip in temperature which releases melatonin [the hormone which regulates our body clock]. But we turn on lights and heating so we're not going to bed as early as we used to.
"Things like TV, computers and the internet probably stop us relaxing and going to bed. Teens and children particularly have this stuff in their bedrooms."
Gurney advises limiting access to TVs, phones and computers in the bedroom - and that goes for adults too.
Other tips include limiting alcohol and caffeine intake, particularly at night, regular exercise, and getting up and going to sleep at the same time each day.
She says weekends are a particularly dangerous time because having a lie in meddles with our body clocks. So it could be goodbye to the much anticipated Shabbat lie-in then.
Tips for a good night's sleep
● Avoid alcohol. It might help put you to sleep but will wake you up later in the night.
● Eliminate late-night caffeine, which makes it difficult for your body to achieve deep, recuperative sleep.
● Incorporate exercise into your daily routine, but do not go for a run too close to bedtime. Recent studies show that regular exercisers sleep deeply and do not wake up at night as often.
● Get up at the same time every day, even when you go to sleep later than usual the night before.
● Try going to sleep in the position you wake up.
● Avoid doing stressful things before bed. It's better to read than to do your accounts.
● Retain your sleep schedule at the weekends. While this might put a kink in your social life, it could mean a big improvement if you have a difficult time sleeping.
● Use your bed for sleeping and sex only. Do not watch television or work there.