Catching some kip in the animal kingdom is not always easy: there are predators waiting to eat you; boisterous young to keep an eye on; and while a tree top may seem like a safe bet for slumber there’s the risk of falling out.
So many animals have found unusual ways of snoozing while ensuring their best chance of survival.
Here are five of the stranger sleeping rituals in the animal world.
Sleeping with one eye open
The idiom "to sleep with one eye open" is a reality for some species.
The phenomenon of sleeping with just one eye closed was most recently discovered in Wahlberg’s epauletted fruit bats (Epomophorus wahlbergi), but also occurs in marine mammals such as common porpoises, bottlenose dolphins and some seals as well as in many birds including domestic chickens, mallards and Humboldt penguins.
Scientists in South Africa say the fruit bats are the first non-marine mammals in which this type of sleeping has been reported, and write that about 21% of bats observed appeared to sleep with one eye open, keeping their closed eye hidden under their wing.
This behaviour is associated with unihemispheric sleep – when one half of the brain shows waking activity while the other shows slow-wave sleep activity.
Unihemispheric sleep is a way of engaging in both wakefulness and sleep at the same time. Animals may do this to avoid predation, to breathe underwater and to allow the brain to recover from sleep deprivation.
Researchers suggest Wahlberg’s epauletted fruit bats might have evolved the strategy to stay alert to predators such as vervet monkeys and African crowned eagles, which have been seen snatching the bats from exposed roosts.
Sea otters' seaweed anchors
Floating on your back to slumber might sound relaxing, but sea otters have to make sure they don’t literally drift off.
The fluffy mammals – owners of the densest fur in the animal kingdon – often wrap themselves in seaweed to stay anchored in sleep. Single sexes often gather in otter "rafts" – groups of as many as 100 sea otters, wrapped in strands of kelp, floating serenely on their backs with tucked up paws on their chest.
And on some occasions sea otters have even been seen holding paws while they sleep, perhaps to stay together.
The mammals grab some shut-eye on land too, but some experts say they prefer to sleep in water, probably because it’s safer.
Sunbirds flaunt their pecs during sleep
Sleeping male malachite sunbird (Nectarinia famosa) may fluff up their bright yellow pectoral tufts to ward away night-time predators.
The colourful birds rarely display their pectoral tufts in the day but scientists at the University of KwaZulu-Natal, South Africa, observed captive birds sleeping with their fluffed up chest tufts throughout the night.
Infra-red recordings revealed the birds displayed continuously through the night.
Any approaching animal might be convinced it is looking into the yellow eyes of a much bigger beast, and stay away.
Orangutans make mattresses
Orangutans carefully construct complex nests including weaving branches and leaves together to make "mattresses".
All great apes can build these sleeping platforms, and are the only other animals apart from humans able to do so.
Nest-building includes bending and breaking large amounts of foliage to shape a sizeable and comfortable platform that bends into a concave shape when slept upon.
A new study showed captive orangutans slept well and deeply in these nests, spread-eagled in a lying-down position on their fronts or backs, compared with baboons, which typically slept upright and on guard.
It takes years for a young orangutan to become an accomplished nest builder.
Previous research has shown they begin practising nest-building at six months old, and are only ready to perform the task successfully at around three or four years.
Why our great ape cousins create these complex constructions is unclear. In the new study, published in the American Journal of Physical Anthropology, researchers suggest they may have begun making nests to avoid falling out of high tree tops. Another possibility, they write, is that they make sleeping platforms "not because they must, but because they can".
No rest for dolphin mums
In humans, a new baby means saying goodbye to normal sleep. But spare a thought for bottlenose dolphin mothers which, for the first few weeks after giving birth, are thought to have almost no sleep at all in order to look after their baby.
Captive dolphins and orcas have been observed not coming to the surface of water to rest for the first three weeks after giving birth. And a study in 2007 showed continuous swimming of mums and babies during this time.
Dolphins are capable of unihemispheric sleep , and scientists have suggested that any episodes of sleep while with a baby are likely to be very brief and fragmented.
This kind of sleep deprivation in the first few weeks of a baby’s life has only been seen in dolphins and orcas. The strategy may be to avoid predators and it allows the mum and baby to make sure they are close to one another at all times.
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