180 Problem 4 You want to buy a motorcycle from JD. The cost is $20,000, $2,000 down and the rest in three equal annual payment beginning one year from today. Interest is included in the payments at 10%. How much are the payments? Amortize the loan. Problem You want to buy a bus. Cost is $160,000 with 10% down and the rest in 5 equal annual payments which include interest at 10%. The payments begin in one year. How much are the payments? Amortize the loan Sleep clears the cluttered brain Gunk between cells is cleansed during slumber By Tina Hesman Saey Sleep showers away cellular grime that its cells to flush out buildup prod- builds up while the brain is awake-just ucts, such as protein pieces that form the sort of process that could have made plaques in people with Alzheimer's dis- sleep a biological imperative, scientists ease, the team had found. After train- reported in October CSN: las ap ing mice to sit quietly on a microscope People have long puzzled over the evo- stage, the researchers could measure lutionary pressures that led animals to the fluid flow while the rodents were need sleep even though it leaves them awake and asleep. Space between cells vulnerable to predators and other dan increased by at least 60 percent when gers. Rinsing off the brain and disposing the animals fell asleep, allowing cere- of waste proteins and other gunk might brospinal fuid to gush in and hose away buildup. When the animals woke up, Manyotherthings that sleep does, such some brain cells-probably ones called as strengthening memories, are impor- astrocytes-swelled up, narrowing the help explain why sleep evolved tant. But they are probably bonuses to crevices separating the cells the real reason that slumber is neces With the drainage system clogged, sary, says Suzana Herculano-Houzel of waste from hardworking nerve cells the Federal University of Rio de Janeiro. begins to pile up. Sleep deprivation or Researchers led by Malken Nedergaard damage to the irrigation system may of the University of Rochester Medi- make it impossible for sleep to fully wash cal Center in New York stumbled upon away the by-products, eventually con- sleep's cleansing function while studying tributing to neurodegenerative disorders how the brain disposes of waste products such as Alzheimer's and Parkinson's The brain pushes fluid in between diseases, the researchers speculate. 24 SCIENCE NEWS December 28,2013