(telegraph.co.uk) In the world of sleep, eight is the magic number. Get eight hours and you should be fully rested for the day ahead. Get seven hours and you're still in a good place. Six, and things start to get tricky.
OK, those numbers vary ever so slightly from person to person, but numerous studies have pointed to the seven-to-eight hour sweet spot.
Which is why a new survey should be enough to keep a lot of us up at night. Compiled by sleep tech firm Simba as part of it's #Tryfor8 campaign, it shows that 44pc of British people get six hours or less. In fact, only 17pc ever manage to hit that magic eight.
It's a problem shared by much of the world. “The frightening thing is that every survey that every country does shows that more and more people are getting less than six hours sleep,” says Professor Paul Gringras, one of the UK’s leading professors of sleep medicine. “Below six hours is where things go wrong. Those people who sleep under six hours have a higher risk of coronary heart disease, their blood pressure is higher and their cholesterol is worse.”
Why does a lack of sleep hurt us? One intriguing area that Gringras is studying is the role of our DNA – and specifically epigenetics.
As most schoolchildren know, DNA is found in every cell in our bodies and contains the instructions for cell replication. Our DNA is basically immutable – so if you have blue eyes, it's hardwired. You can't change them to brown.
However, DNA isn't the whole story, and this is where epigentics come in. These are signals which tell our DNA what to do and when. Should cells keep dividing? Should they produce a certain kind of protein to help perform a specific function? Epigenetics switch DNA on and off – and unlike the DNA itself, they're affected by environmental factors. If you don't get six hours of sleep, your DNA stays the same, but your epigenetics go a quiet form of haywire.
Gringras gives the example of clock genes, which ensure we produce certain cells at certain times of day. If you lose sleep – say, by going on a long distance flight – your circadian rhythm gets thrown out of sync, changing your epigenetic signals to the genes. And that could mean sending a message to increase weight gain.
“There’s a change in DNA methylation, one of the switches that we know about, which causes adipose tissue (basically fat) to increase its ability to store fat, and muscle protein to breakdown,” says Gringras. “In addition, we know sleep affects production of two hormones, ghrelin and leptin. Ghrelin is the one which drives the appetite in an unhealthy way.”
Epigenetics aside, there are also significant cognitive changes that start to happen in our brains after less than six hours sleep. Specifically, we start to have trouble processing memories.
The first important thing to understand is that not all memories are the same. We have working memory, which is the space that manipulates and holds information before it's either forgotten or gets encoded into long-term memory. Think of it like the copy function on your computer. Once you turn off your computer, whatever you last copied is gone forever.
But your brain can paste the data into a document and 'save’ it in long-term memory. If you then need to use that memory, you have to transfer it back from long-term to working memory, essentially opening the document and hitting the copy function again.
The hippocampus is the part of the brain which makes the decision about whether to retain memories or clear them. And this happens when we're sleeping.
“Deep sleep makes things more stable and then integrates it all with other memories,” explains Gringras. “It's almost like you're rehearsing all the things that happen during the day, and long-term memories get formed really, really tightly. They can stay for the whole of your life.”
If we don’t get enough sleep the hippocampus can’t function properly and working memories don’t necessarily get transferred into long-term memory. If your brain continues to deal with a lack of sleep over the long term, sleep scientists now believe you’re more likely to be at risk of dementia and Alzheimer’s disease.
And then there's the more recognisable cognitive effects of sleep deprivation: waning concentration and a general feeling of grogginess.
“People's vigilance deteriorates," says Dr Guy Leschziner of the London Bridge Sleep Centre. "What we think might be happening is that the more sleep deprived we are, the stronger the drive to sleep is, and therefore small areas of the brain might be dozing off while we are apparently awake.
“So when we put people through sleep studies, or when we monitor their wakefulness during sleep deprivation, we see something called microsleeps. These are very, very brief periods of sleep in the whole of the brain, but we also think there may be little areas of the brain that might be constantly dipping in and out of sleep.”
Alongside this, there's the fact that losing even an hour’s sleep makes you more like to suffer from heart attacks or strokes. According to a study from 2014, the day after day British Summer Time begins in spring, hospitals report 24pc more heart attacks across the UK. (On the opposing day in autumn, hospitals see an average of 21pc less heart attacks.)
“Sleep is important for healing, for regulation of the immune system, regulation of part of the nervous system called the autonomic nervous system; the part that's responsible for temperature control,” explains Leschziner. “And also there are significant changes in terms of inflammation of the body which is why sleep might be associated with things like strokes and heart disease.”
The good news, says Gringras, is that the body recovers quickly from sleep deprivation – so long as it's not frequent. "We know that people regularly getting less than six hours, their blood pressure goes up, their risk of strokes goes up, their risk of Alzheimers goes up, and their obesity rates are higher. So it definitely hits, but I wouldn't panic if you get one bad night's sleep.”
