“The purposes and mechanisms of sleep are only partially clear and are the subject of intense research” Source Wikipedia
Although extensive research in humans and animals has shown unequivocally that sleep is essential, the reasons why it is required are as yet unknown. Long term sleep deprivation not only severely impairs cognitive and motor skills, it's now been shown that it can cause physical damage to brain structure - and, in extreme cases, irreversible damage.
From an evolutionary point of view, laying unconscious for extended periods would presumably have made our ancestors vulnerable to attacks, so the necessity of sleep must outweigh the considerable dangers it presents.
There are a large number of theories regarding the function and need for sleep - none has yet been generally accepted as a full explanation.
• Memory consolidation
• Waste chemical removal
• Energy conservation
To quote William Dement, founder of Stanford University's Sleep Research Center,
“As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.” (National Geographic Magazine, May 2010) (paywall)
A sleep phenomenon known as 'unihemispheric sleep' - i.e. the ability for just one half of the brain to engage in sleep (while the other half remains fully awake) - has been observed in numerous species, including birds, reptiles and aquatic mammals (e.g. cetaceans, eared seals and manatees).
The reasons why no land mammals (as far as is known) cannot unihemispherically sleep is not known.
“The relative absence of unihemispheric sleep in mammals suggests that a trade off exists between unihemispheric sleep and other adaptive brain functions occurring during sleep or wakefulness. Presumably, the benefits of sleeping unihemispherically only outweigh the costs under extreme circumstances such as sleeping at sea.” source
The American Academy of Sleep Medicine (AASM) and Sleep Research Society (SRS) have developed a consensus recommendation for the amount of sleep needed to promote optimal health in adults, using a modified RAND Appropriateness Method process, and based on more than 4,000 previously published studies. They conclude that :
“Sleeping less than 7 hours per night on a regular basis is associated with adverse health outcomes, including weight gain and obesity, diabetes, hypertension, heart disease and stroke, depression, and increased risk of death. Sleeping less than 7 hours per night is also associated with impaired immune function, increased pain, impaired performance, increased errors, and greater risk of accidents.
Sleeping more than 9 hours per night on a regular basis may be appropriate for young adults, individuals recovering from sleep debt, and individuals with illnesses. For others, it is uncertain whether sleeping more than 9 hours per night is associated with health risk.”
There is currently no explanation as to why some mammals, like humans, appear to need about 8hrs of sleep in each 24hr period - some other mammals routinely sleep 18hrs, and others seem to need only 3hrs or so. [examples needed]