Why do donkeys snooze for just three hours a day, while hairy armadillos are knocked out for more than 20? Biologists have struggled to find any satisfactory explanation for the bewildering variation in how much different mammals sleep.
However, new studies that take evolutionary relatedness into account promise to revolutionise the field. In particular, one large study suggests that REM (rapid eye movement) sleep 鈥� during which the brain is highly active 鈥� may play a key role in intelligence.
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Lab studies in humans already suggest that REM sleep is important for cognitive abilities such as consolidating memories 鈥� a good night鈥檚 sleep 鈥� with plenty in the REM phase 鈥� can improve people鈥檚 ability to remember what they have learned in the day by about 15%.
In other species, the evidence is less clear cut. If REM sleep helps learning, then mammals with more developed brains should presumably need more of it, but in the past no such relationship has been found.
Fatal flaw
One of the few biological functions that has been found to correlate with sleep patterns is metabolic rate. Animals with a relatively high metabolic rate for their body size seem to need more non-REM sleep, suggesting that catching extra Zs simply helps them conserve precious energy.
But of the Max Planck Institute for Ornithology in Starnberg, Germany, believes these studies had a fatal flaw 鈥� researchers were comparing species without taking their evolutionary relationships into account.
He says that big differences in sleep patterns between evolutionary groups can swamp relationships found within those groups (see graph, below right).
So Lesku and his colleagues compiled studies looking at sleep patterns in 83 species of mammals 鈥� from opossums and sloths, to cows, beavers, macaques and people 鈥� and reanalysed the data using statistical techniques to account for their position on the evolutionary tree.
鈥淭hese techniques are standard throughout biology,鈥� he says, 鈥渂ut for some reason they never permeated sleep research.鈥�
鈥楥all to arms鈥�
The results contradict previous work. Once evolutionary relationships were factored in, the data showed that animals with big brains for their body size need a significantly higher percentage of REM sleep 鈥� supporting a role in intelligence and cognitive function.
And species with high metabolic rates for their size needed less non-REM sleep, not more. This suggests that they don鈥檛 sleep to conserve energy. Instead, animals with high metabolic rates may sleep less because they burn more calories, so have to spend more time foraging for food.
Lesku鈥檚 results first appeared in . But since then support has been growing for the idea that sleep simply keeps animals immobile and out of harm鈥檚 way.
So this month he will publish a broader discussion of his results in Sleep Medicine Reviews, as a 鈥渃all to arms鈥� for the field. 鈥淚t鈥檚 to emphasise the necessity for these kinds of research,鈥� he told New 杏吧原创. 鈥淓volution does matter.鈥�
鈥楽trongest signal鈥�
Lesku鈥檚 point of view is supported by by Isabella Capellini of Durham University, UK, and colleagues, who tested the strength of the influence that evolutionary relatedness has on sleep patterns 鈥� and found it to be highly significant.
鈥淪leep scientists have ignored the fact that sleep could be affected by evolutionary relationships,鈥� says Capellini. 鈥淏ut it鈥檚 the strongest possible signal.鈥�
After taking evolution into account, Capellini鈥檚 team found that ecological factors are more important than previously acknowledged. For example, species at risk of predation tend to sleep less. 鈥淧rey can鈥檛 afford to sleep for longer,鈥� she says. 鈥淭his indicates that if they still sleep at all, it must do something important.鈥�
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