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Sleeping While Awake Lant

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Sleeping While Awake Lant CONSCIOUSNESS REDUX “It was literally true: I was going through life asleep. My body had no more feeling than a drowned corpse. My very existence, my life in the world, seemed like a hallucination. A strong wind would make me think my body was about to be blown to the end of the earth, to some land I had never seen or heard of, where my mind and body would separate forever.” —From Sleep, by Haruki Murakami, 1989 PHYSIOLOGY turn off—your mind remains hypervigi- Sleeping While Awake lant. You toss and turn but can’t find the blessed relief of sleep. The reasons for During microsleep, the entire brain nods off so briefly that we often don’t notice it. sleeplessness may be many, but the con- ) Now research shows that individual neurons in the brain can slumber, too, sequences are always the same: You are Koch fatigued the following day, you feel especially when we are sleep-deprived E ( sleepy, you nap. Attention wanders, your B CA We’ve all been there. You go to bed, reaction time slows, you have less cogni- C close your eyes, blanket your mind and tive-emotional control. Fortunately, BY CHRISTOF KOCH wait for consciousness to fade. A timeless fatigue is reversible and disappears after ); SEAN M Christof Koch is president interval later, you wake up, refreshed a night or two of solid sleep. and chief scientific officer at and ready to face the challenges of a new We spend about one third of our lives the Allen Institute for Brain day (note how you can never catch your- in a state of repose, defined by relative illustration Science in Seattle. He serves on Scientific American self in the act of losing consciousness!). behavioral immobility and reduced re- N HAN ( O Mind’s board of advisers. But sometimes your inner world does not sponsiveness to external stimuli. Cumu- J 20 SCIENTIFIC AMERICAN MIND NOVEMBer/DecemBer 2016 Exploring the riddle of our existence latively, this amounts to several decades’ as fast as four times a second (that is, in pants had 79 microsleep episodes per worth of sleep over the lifetime of an av- the 0.25- to four-hertz frequency range). hour, lasting between 1.1 and 6.3 sec- erage person. Ah, I know you’re think- Tuning into the discharge of individual onds apiece, with an attendant drop in ing, Wouldn’t it be great if we cut down neurons during deep sleep reveals dis- performance. Microsleep shows up in on this “wasted” time to be able to do crete off periods, when nerve cells cease the EEG record by a downward shift more! When I was younger, I, too, lived generating any electrical activity for 300 from activity dominated by the alpha by the motto “You can sleep when you’re to 400 milliseconds. Such recurring si- band (8 to 13 Hz range) to oscillations dead.” But I’ve woken up to the fact that lent periods, synchronized across large in the theta band (4 to 7 Hz). for optimal, long-term physical and mental health, we need sleep. Humans share this need for daily sleep with all multicellular creatures, as MICROSLEEP CAN BE FATAL WHEN DRIVING OR anybody growing up with dogs, cats or OPERATING MACHINERY SUCH AS TRAINS OR other pets knows. AIRPLANES, HOUR AFTER TEDIOUS HOUR. An understanding of sleep’s impor- tance can be observed by contemplat- ing the biological process itself. Sleep is homeostatically regulated with exqui- parts of the cortex, are the cellular hall- Perniciously, subjects typically be- site precision: pressure to go to sleep mark of deep sleep. lieve themselves to be alert all the time builds up during the day until we feel during microsleep without recalling any sleepy in the evening, yawn continuous- Microsleep period of unconsciousness. This misap- ly and nod off. If deprived of sleep, hu- My last column, “To Sleep with Half prehension can be perilous to someone mans experience an ultimately irresist- a Brain,” highlighted the growing real- in the driver’s seat. Microsleep can be fa- ible need to seek repose—they, in fact, ization of sleep researchers that being tal when driving or operating machinery become “sleep drunk.” An older, 19th- awake and asleep are not all-or-none such as trains or airplanes, hour after te- century term, closer to the truth, is “ce- phenomena. Just because you’re asleep dious hour. During a microsleep epi- rebral exhaustion,” the brain demand- doesn’t necessarily imply that your en- sode, the entire brain briefly falls asleep, ing its rest. tire brain is asleep. Conversely, as I will raising the question of whether bits and In my last Consciousness Redux col- describe now, we have also learned that pieces of the brain can go to sleep by umn, I described how clinicians define even when you’re awake, your entire themselves, without the entire organ sleep by recording brain waves from a brain may not be awake. succumbing to slumber. net of electroencephalogram (EEG) sen- A case in point for sleep intruding Indeed, Italian-born neuroscientists sors placed on the scalp of the sleeper into wakefulness involves brief episodes Chiara Cirelli and Giulio Tononi, who [see photograph on next page]. Like the of sleep known as microsleep. These in- study sleep and consciousness at the surface of the sea, the electrical brain is tervals can occur during any monoto- University of Wisconsin–Madison, dis- ceaselessly in commotion, reflecting the nous task, whether driving long distanc- covered “sleepy neurons” in experimen- unseen, tiny tremors in the cerebral cor- es across the country, listening to a tal animals that showed no behavioral tex underneath the skull that are picked speaker droning on or attending yet an- manifestation of sleep. In this research, up by the EEG electrodes. Rapid eye other never-ending departmental meet- 11 adult rats had microwires implanted movement (REM) sleep is characterized ing. You’re drowsy, your eyes get into their frontal motor cortex, which by low-voltage, choppy, swiftly chang- droopy, the eyelids close, your head re- controls movement. Inserted into the ing brain waves (paradoxically, also typ- peatedly nods up and down and then cortical tissue, the sensors picked up ical of relaxed wakefulness), whereas snaps up: your consciousness lapses. both the voltage called the local field po- non-REM sleep is marked by slowly ris- In one experiment attempting to ex- tential (LFP), akin to the EEG, in addi- ing and falling waves of larger ampli- plore this condition, participants had to tion to the spiking activity of nearby tude. Indeed, the deeper and more rest- track a randomly moving target on a nerve cells. As expected, when awake, ful the sleep, the slower and larger the computer monitor with a joystick for 50 the LFP was dominated by low-ampli- waves that reflect the brain’s idling, re- minutes. While straightforward, this vi- tude, fast waves readily distinguishable storative activity. These voltage oscilla- suomotor task demands nonstop atten- from the larger and slower waves char- tions, referred to as delta waves, can be tion that becomes difficult to sustain af- acteristic of non-REM deep sleep [see as slow as once every four seconds and ter a while. Indeed, on average, partici- box on page 23]. MIND.SCIENTIFICAMERICAN.COM SCIENTIFIC AMERICAN MIND 21 CONSCIOUSNESS REDUX As expected from pre- One question was whether any one vious animal and human neuron fell asleep independent of any studies, by the end of the other neuron. Or was this occurrence sleep deprivation phase, more of a global phenomenon, whereby the LFP began to shift all neurons simultaneously transition to lower frequencies, to an off period? The answer, obtained compatible with the idea by implanting a second array of micro­­ that the pressure for the wires into a second cortical region—the animals to sleep steadily parietal cortex, a quite distinct region built up. Closer inspection from the motor cortex—was “yes” to of the electrical signatures, both questions. however, revealed some- That is, sometimes neurons in both thing unexpected: occa- regions went off together, whereas at sional, spo radic, silent other times they did so independently. periods of all or most of Yet as the sleep pressure built up, after the neurons in the record- several hours of being kept awake, neu- ed brain region [see box ronal activity during sleep deprivation on opposite page] without did become more globally synchronized the animals showing ei- (as it does in deep sleep). Likewise, the ther behavioral or EEG longer the animal slept during the recov- manifestations of micro- ery period, the less likely slow waves sleep. These short, off- were simultaneously detected at both The author, his head clad in a dense net of EEG sensors, par- like episodes were often cortical sites. Groups of neurons can be ticipates in a sleep study in the laboratory of Chiara Cirelli associated with slow more easily recruited to produce the and Giulio Tononi at the University of Wisconsin–Madison. waves in the LFP. The op- slow oscillations that constitute deep posite happened during sleep when sleep pressure is high. At the level of individual neurons, recovery sleep, toward the end of this These results paint a more nuanced the awake animals’ cortical cells chat- six-hour period, when the pressure to view of wakefulness and sleep than the ted away in an irregular, staccato man- sleep had presumably abated. At this prevailing one, in which both conditions ner over an extended period. Converse- point, large and slow waves in the LFP were considered to be global, all-or-none ly, during deep sleep, cortical neurons became more infrequent, and neuronal states of consciousness.
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