Sleep tight

Controversies over why animals sleep

source: Emma Young
New Scientist vol 197 no 2647, March 15 2008
starts p 34, 5 pages long

Researchers disagree on why animals sleep. Some researchers see sleep as having a biological function. University of California, Los Angeles' Jerry Siegel, argues that sleep may have evolved to protect animals and conserve energy.

REM, or rapid eye movement, sleep, when the brain is active, is linked to memory improvement, and seeing patterns and rules, according to studies with humans. Dolphins, however, may not experience REM sleep, Siegel notes. Meanwhile, Oleg Lyamin, from Russia's Utrish Dolphinarium in Moscow, has found that fur seals use just one hemisphere for sleeping, at any given time, and have very low REM levels, during their hunting expeditions at sea, but their sleep patterns change to resemble those of other mammals their size when they return to land. They also do not experience REM rebound, unlike dogs or humans deprived of REM sleep.

Siegel argues that a possible function of REM sleep is to permit animals to wake up more alert than if they were woken when not in REM sleep. REM could also, he suggests, help keep brainstems active, so would not be needed by dolphins with one hemisphere awake. Meanwhile, Chiara Cirelli, from University of Wisconsin Madison's Center for Sleep and Consciousness, argues that non-REM sleep is important in moderating synaptic connections in mammals and birds.

Siegel does not discount biological functions of sleep, but argues that it is unlikely to perform a key function. He notes that bullfrogs do not sleep. Giraffes sleep very little, and lions sleep some 14 hours daily. John Lesku, from Germany's Max Planck Institute for Ornithology, Starnberg, has discovered that larger brains tend to be liniked to higher levels of REM sleep, and that sleep patterns tend to be similar between species that have close genetic relationships.

Siegel argues that being awake is more risky than being asleep. Bats that eat flies which can only be found for a few hours a day tend to sleep the rest of the time. Isabelle Capellini, from England's University of Durham, sees sleep as having evolved for biological reasons, with ecological factors affecting how long animals sleep. Siegel, on the other hand, places more importance on ecological than on biological factors.