We spend a third of our lives doing it.
Now scientists have finally discovered why we need to spend so much time sleeping - it helps clear the mind of the day�s chemical clutter.
While our body is at rest, the brain is hard at work removing toxins produced during our waking hours.
Left to build up, these compounds can result in Alzheimer�s and other neurological diseases.
The researchers conclude the clean-up process is so energy intensive, it would hinder our thinking if done when we are awake - hence the need to sleep.
�This study shows that the brain has different functional states when asleep and when awake,� said lead researcher Dr Maiken Nedergaard, from the University of Rochester Medical Centre (URMC), in New York.
�In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness.�
The purpose of slumber has been debated for centuries, with Thomas Edison branding it �a criminal waste of time.�
And although practically every species needs to sleep, many have suggested it is a faulty evolutionary hang-up that makes us more vulnerable to predators.
The team found that unlike the rest of the body, which depends on the lymphatic system to drain away toxins, the brain has its own separate method of rubbish removal.
Scans on mice revealed the amount of energy used by the brain did not dramatically fall during sleep.
This is because its cleaning activities increased 10-fold at times of rest, according to the researchers, who observed that significantly higher levels of toxins were also removed.
The amount of brain power needed to clean up toxins means we would be unable to think clearly if the process took place during waking hours, the team speculated.
The findings, published in the journal Science, go some way to explain the biological purpose of sleep and could lead to new ways to treat neurological disorders.
�The brain only has limited energy at its disposal and it appears that it must choice between two different functional states - awake and aware or asleep and cleaning up,� said Dr Nedergaard.
�You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can�t really do both at the same time.�
And to aid the clean-up operation, scientists found brain cells can shrink by up to 60 per cent during slumber, to allow waste to be removed more effectively.
They also observed that a hormone called noradrenaline is less active during sleep, suggesting it could be controlling the contraction and expansion of the brain�s cells during sleep-wake cycles.
�These findings have significant implications for treating �dirty brain� diseases like Alzheimer�s,� said Dr Nedergaard.
�Understanding precisely how and when the brain activates the glymphatic system [brain�s cleaning system] and clears waste is a critical first step in efforts to potentially modulate this system and make it work more efficiently.�
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