How sleep clears the brain of toxins "This study shows that the brain has different functional states when asleep and when awake," said Dr Maiken Nedergaard, a researcher from the University of Rochester in the US. 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 findings, published in the journal Science, show that the brain's unique method of cleansing itself - known as the glymphatic system - is highly active during sleep. As we slumber, it clears away toxins that would otherwise build up and trigger neurological disorders such as Alzheimer's disease. The scientists also found that during sleep the brain's cells reduce in size to allow waste to be removed more effectively. Studies in mice revealed how the brain's glymphatic waste disposal system works, by pumping cerebral spinal fluid through brain tissue and flushing toxins into the blood circulation and liver. Scientists speculated that the cleaning process may not be compatible with functions that the brain must perform while awake. "The brain only has limited energy at its disposal and it appears that it must choose between two different functional states," 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.