Drifting off for forty winks can be wonderfully restorative, but Ryan Tisdale from the Max Planck Institute of Ornithology has misgivings about the wisdom of nodding off. ‘From an evolutionary standpoint, sleep is a baffling state’, he says, outlining how sleepers fritter away valuable time, in addition to leaving them vulnerable to predators. ‘Animals try to reduce the risk of predation by retreating to safer sleep sites whenever possible and by sleeping during the safest time of day’, says Tisdale. Knowing that mammals tweak their sleep patterns depending on their sense of safety and that slumbering birds prefer higher roosts, although they will sleep nearer to the ground if necessary, Tisdale and an international team of collaborators wondered whether pigeons adjust their sleep patterns depending on where they choose to lay their heads.
‘Sleep can be … divided into two distinct sleep sub-states’, says Tisdale, explaining that birds and mammals switch between rapid eye movement (REM) sleep – where the eyes twitch, the muscles relax and the brain is active, but the animal is less responsive to its surroundings – and slow-wave sleep – also known as non-REM sleep – over the course of a night. After attaching electrodes to the heads of six pigeons (Columba livia), Tisdale recorded the animals’ brain waves over the course of a night when the pigeons had the choice of sleeping near the ground (20 cm) on perches or high up (170 cm). Then, on the following night, the team removed the lofty perches, forcing the pigeons to slumber out of their comfort zone, before reinstating the taller perches the following evening.
Comparing the brain wave patterns over the three nights, it was clear that the birds slept more lightly on the night when they were on a lower-level perch than when they had access to more secure, higher perches. They slipped into REM sleep less often and only dipped lightly into slow-wave sleep, although they did not cut back on the length of time passed in slow-wave sleep. However, when the team returned the secure perches, the animals indulged in more REM sleep than they had on either of the previous nights, suggesting that they were catching up on the lost REM sleep.
Tisdale says, ‘Their results parallel those observed in mammals after encountering threatening situations’. He also suspects that REM sleep is particularly dangerous because it is difficult to arouse animals from it, leaving them vulnerable to attack. However, he concludes that it must be important for some reason, otherwise ‘evolution should have done away with it a long time ago’, he says.
