Although sleep is universal, researchers still haven't been able to figure the function of sleep. Or how it works.
While many theories suggest that sleep helps in brain "maintenance" - including memory consolidation and pruning- reverse damage from oxidative stress suffered while awake and promote longevity, none of them are well established.
Now, researchers from University of California, Los Angeles have come up with a new theory that sleep's primary function is to increase animals' efficiency and minimize their risk by regulating the duration and timing of their behaviour.
"These behaviours include eating, procreating, caring for family members, monitoring the environment for danger and scouting for prey.
"So it's been thought that sleep must serve some as-yet unidentified physiological or neural function that can't be accomplished when animals are awake," he added.
In the study conducted using platypus, walrus, and echidna - a small, burrowing, egg-laying mammal covered in spines, the researchers showed that sleep itself is highly adaptive, much like the inactive states seen in a wide range of species, starting with plants and simple microorganisms; these species have dormant states - as opposed to sleep - even though in many cases they do not have nervous systems.
That challenges the idea that sleep is for the brain, said Siegel.
"We see sleep as lying on a continuum that ranges from these dormant states like torpor and hibernation, on to periods of continuous activity without any sleep, such as during migration, where birds can fly for days on end without stopping," he said.
In humans, the most notable thing about sleep is that it reduces body and brain metabolism while still allowing high level of responsiveness to the environment, such as parent arousing at a baby's whimper but sleeping through a thunderstorm.
"This Darwinian perspective can explain age-related changes in human sleep patterns as well," said Siegel.
"We sleep more deeply when we are young, because we have a high metabolic rate that is greatly reduced during sleep, but also because there are people to protect us.
"Our sleep patterns change when we are older, though, because that metabolic rate reduces and we are now the ones doing the alerting and protecting from dangers," the expert added.
The study appears in journal Nature Reviews Neuroscience.