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The 7 hours of sleep we typically get every night often doesn鈥檛 feel like enough. Compared with our fellow primates, which spend around 12 hours of each day slumbering, humans barely get any shut-eye.

It seems we have evolved to limit how long we sleep, and that may simply be because we have more important things to do with our time, says at Duke University in North Carolina. However, the trade-off might have left us more susceptible to Alzheimer鈥檚 disease.

Nunn and his colleague collected data on how long 29 other primate species sleep, including how much time they spend in the REM (rapid eye movement) sleep associated with vivid dreams and in deep, non-REM sleep.

The pair also included various factors that might influence an animal鈥檚 sleep, such as diet, brain size and whether it lives in groups. They used all this to build a model that predicts how much sleep a given primate species should get, based on the others.

Evolved not to sleep

Their model predicted that humans should get around 9.5 hours of sleep every night. Of this, 8.4 hours would be in non-REM sleep and 1.3 hours in REM sleep 鈥� if we were like other primates.

In reality, we typically get just under 7 hours of sleep a night, spending 5.4 hours in non-REM sleep, and 1.6 hours in REM. So the reduction in how long we sleep has come about by cutting non-REM sleep.

The findings don鈥檛 mean we should be slumbering for 9.5 hours, says Nunn. 鈥淲e know that, in terms of health, there is an optimal amount of sleep, and people who sleep more or less than that have health problems,鈥� he says. 鈥淚t definitely seems that sleep has been optimised.鈥�

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Nunn thinks humans sleep less because we have more important things to do. The time our ancestors spent sleeping could have been used to do all the things that made humans so successful, such as finding allies, developing skills like tool making and teaching our children, he says.

But not everyone is convinced. Other primates have plenty of learning to do, says at the University of Lincoln, UK. Chimpanzees procure a variety of foods, make tools, have complex social lives and occasionally fight wars 鈥� yet they sleep much more than we do, he says.

Durrant also points out that the study data came from animals living in captivity, which probably sleep more than their wild counterparts. 鈥淭here鈥檚 increased danger in the wild, and animals in captivity are notoriously bored,鈥� he says.

Alzheimer鈥檚 risk?

Nunn and Samson aren鈥檛 sure why we have cut non-REM sleep and preserved REM sleep. Non-REM sleep seems particularly important for storing long-term memories.

There also appears to be a link between the amount of non-REM sleep a person gets and whether or not they develop Alzheimer鈥檚 disease. Some have suggested that the plaques of amyloid protein that characterise this condition are cleared from the brain in non-REM sleep. 鈥淧erhaps this may make humans uniquely vulnerable to Alzheimer鈥檚 disease,鈥� says Nunn.

Such a change in sleeping patterns might indeed have increased this risk, agrees at Washington University in St Louis, Missouri. Our brains make less of the amyloid protein characteristic of Alzheimer鈥檚 during non-REM sleep. 鈥淚f you shorten the period of non-REM sleep, presumably levels of the protein are higher, allowing it to accumulate over time,鈥� says Holtzman.

That said, sleeping over and above the average 7 or so hours won鈥檛 necessarily protect you against Alzheimer鈥檚, given that research shows that oversleeping can also harm your health.

鈥淚 don鈥檛 think we can improve on someone who gets normal amounts of sleep,鈥� says Holtzman. 鈥淏ut many people don鈥檛 have appropriate sleep patterns. It is more important to correct those to protect against problems.鈥�

He recommends getting enough exercise, not reading too close to bedtime and switching off lights when you sleep.