Australasian Science: Australia's authority on science since 1938

In Search of Lost Sleep

By Tim Olds

Are we getting less sleep than we did in the past? And how much do we really need?

Try this: google “NGrams”, which searches millions of digitised books to track historical changes in the frequency of words and phrases. Now type in “not enough sleep”. You will see that the frequency of this phrase has risen from 46 per billion words in 1921 to a temporary high of 5995 per billion in 1944 (sleepless nights during the Second World War), plunging again to 1532 in 1967 (the summer of love), only to skyrocket to 7059 in 2000 – 153 times higher than the 1921 figure.

Sleep is on our minds more than ever. Sleep is like the dark matter of the use-of-time universe: we know it’s important, we know there must be more of it out there somewhere, but we’re not seeing it.

There’s nothing much to be said for poor sleep. It affects our attention, memory, creativity, learning and academic performance. It makes us more emotional, increasing our impulsivity, aggression and hyperactivity. Kids who sleep badly are at greater risk of alcohol and drug abuse and suicide risk as adults. Poor sleep compromises our immune systems, and increases our risk of overweight and obesity. Kids who don’t get enough sleep have trouble learning motor skills like serving a tennis ball.

So is it true what they say? Are we getting less sleep than we did in the past? Researchers from the University of Sydney looked at a series of adult time use studies across 15 countries, and found no evidence of declines in sleep between the 1960s and today.

But the story seems a bit different with kids. Lisa Matricciani of the University of South Australia analysed data on almost 700,000 children from 20 countries over the 20th century, and found that kids have been losing sleep at the rate of about 45 seconds each year. It doesn’t sound like much, but that means that your kids are probably sleeping about 25 minutes less each night than you did when you were their age, and you in turn were sleeping 25 minutes per night less than your parents. That’s a loss of about 300 hours of sleep each year over two generations.

What has made this possible is the killer combination of electricity and caffeine. Caffeine allows us to keep awake, and electricity gives us something to do. Going back to the days before electric lights and caffeine, sleep was a very different affair indeed.

Social historian Roger Ekirch provides evidence that we used to sleep in two shifts, rather than sleeping through the night. We would get up around midnight for the pre-industrial equivalent of a cup of coffee and a World Cup footy game (perhaps a cup of mulled wine and some communal spinning) before going back for a second dose.

Of course, the fact that kids are sleeping less doesn’t mean that they’re not getting enough sleep. Maybe our parents just luxuriated in unnecessary sleep.

British sleep researcher Jim Horne is very much of this opinion, at least as far as adults are concerned. Sure, we’ll all take a bit more sleep if it’s on offer, but we’d all have a bit more food, too – and that’s not good for us.

In experimental studies, the evidence is reasonably consistent, at least when sleep is dramatically curtailed or extended. Sleep extension studies, where people are encouraged to sleep as long as they like (and get paid for it), almost always result in better performance on cognitive tasks, and better blood chemical and hormonal balance.

Conversely, cruel experiments where people are forced to endure curtailed sleep show the opposite: levels of the stress hormone cortisol go up, blood sugar levels are all over the shop, levels of the appetite-stimulating hormone ghrelin rise. So it looks like most of us would benefit from some extra sleep.

But how much do we need? Matricciani also produced a fascinating study of the history of sleep recommendations for kids, again going back 100 years. She found that the recommended amount of sleep has been going down for the past century at exactly the same rate that sleep has been declining. But the recommended amount of sleep has always been about 40 minutes more than kids were getting, no matter how much they were getting.

So how much sleep do kids need? We really haven’t a clue, but we still suspect it’s more than they’re getting.

Professor Tim Olds leads the Health and Use of Time Group at the Sansom Institute for Health Research, University of South Australia.