十 」 E MINDTRICKS: Researchers believe synapses ⅲ the brain are pared down during sleep, which strengthens important ー第 e ーれ 0- ries and clears ⅳ「 e ト evant infO, among other things. microscopy, which produces extremely high- resolution images Of neurons within tWO areas ofthe mouse cerebral cortex, a part ofthe brain uses a tiny knife tO slice intO the brain, scans a layer Of neurons and repeats this process, pro- ducing thousands 0f images. The team then analyzed this bevy of data to record the behav- ior ofsynapses day and night. For more than a decade, the team has hypothesized that the brain would behave this way, a theory called the "synaptic homeostasis hypothesis. ” They suggested the strengthening Of synapses that occurs When one IS awake can- not happen indefinitely; there is only so much space in the brain, and some sort Of reset iS needed. This study provides some 0f the first concrete evidence that structural changes take place during slumber and at night, says Derk- Jan Dijk, a professor of sleep and physiology at the University ofSurrey, whO wasn t involved in the research. Maiken Nedergaard, a scientlst at the Univer- sity ofRochester Medical Center wh0 also wasn t involved in the paper, says these discovenes "show that the different states Of brain activity actually alter [its] structure.... That is an almost scary thought. " A second paper ⅲ S ロ , by mchard Huganir, Graham Diering and colleagues 仕 om J0hns HOP- kins University, provldes insight int0 the bio- chemistry controlling this process. The group found that a protein called Homena inserted itself intO synapses when mice got very tired and dozed 0 initiatmg a chain 0f events that led t0 the we akening Of conne Ct1011S With Other neu- rons. Likemse, during wakefulness this protein was expelled from synapses, and it appears t0 be prevented from binding by the neurotransmitter noradrenaline, WhiCh iS associated With alertness and drops during sleep, Diering says ・ The results suggest sleep has a restorative func- tion at a structural level, 。 like a sort Of wiping Of the slate," Diering says ・ B0th studies help explain why sleep deprivation lmpairs れ ory. Dienng notes that When mice were overtired and unstimulated, Homena was found in high levels in their synapses, suggest- ing it had already begun the scaling-down pro- cess assoclated withsleep, ike y mterfermg W1th e armng a ⅱ d はれ 0n10 一 CO ⅱ 901id tion. Adeno sine also plays a role. This neurotrans- mitter builds up the longer one is awake and n n t s that caffeine is thought t0 work by blocking adenosine 's activity and preventmg it 仕 om causing drowsmess. When the team gave mice a stimulant similar tO caffeine' Homer1a didn't enter intO synapses and begin tO do its thing, he adds. More research needs tO be done tO ShOW con- clusively what happens during sleep, however. While "these studies represent extremely lmport- ant advance s in the scientific e ffort tO solve the mystery of sleep, ” there could be other expla- nations for What iS causing these changes, says Marcos Frank, a neuroscientlst at Washington State University. He notes that the brain has a master clock that controls many biological func- tions, which could play a role in this downscaling process independent 0f sleep. He's also skeptical SLUMBER IS VITAL FOR CLEARING OUT TOXIC WASTE FROM THE BRAIN. that other areas Of the brain—aside from the two areas Ofthe mouse cortex investigated in the WiS- consin-led paper—will behave similarly. Nedergaard says the synaptic homeostasis hypothesis is "the best theory out there" but stresses that sleep has Other important functions. For example , her work has shown that slumber is vital for clearing out tOXIC waste from the brain. The studies suggest that the paring down that happens during sleep isn't uniform—alre ady strong synapses that likely help store import- ant 1 れ ones aren't touched and in some cases appear tO be strengthened. About 20 percent Of the synapses aren t scaled down. We suggest these are the ones [associated withl your very stableknowle dge/likeyour mem- oriesof your momand hername,"GellÜsays. "That's not something you'll forget no matter What. ” Lesser important connections, however, ma be cleared. And that's a good thing. "Tomorrow IS a new day," Hugamr says. Someb0dy ought t0 remind Punxsutawney Phil. ロ N E W W 0 R L D / S L E E P NEWSWEEK 49 MARCH 24 , 2017