National Geographic

So are the parts of the brain that generatemotion—which is why there’s frequently asense of flying or falling in dreams. We dream,as well, in full color, unless we’ve been blindfrom birth, in which case dreams do not havevisual imagery but remain emotionally intense.Men’s and women’s dreams seem to be similarin emotional content. Every time a man dreams,even if the content isn’t sexual, he has an erec-tion; in women, blood vessels in the vaginaare engorged. And while we dream, no matterhow absurd, despite all transgressions againstthe laws of physics, we’re almost always con-vinced we’re awake. The ultimate virtual-realitymachine resides inside our head.Thank goodness we’re paralyzed. When youdream, your brain is actually trying to producemovements, but a system in the brain stemcompletely shuts down the motor-neuron gate.There’s a parasomnia—a sleep abnormalitythat afects the nervous system—called REMbehavior disorder in which the gate does notfully lower, and people act out their dreams inspectacular fashion, punching, kicking, swear-ing, all while their eyes are closed and they’refully asleep. This often results in injuries to thesleeper and his or her bedmate.The end of a REM session, like the end of stage4, is usually marked with a brief awakening. Ifwe rest naturally, without an alarm clock, ourlast dream of the night often concludes oursleep. Though the amount of time we’ve beenasleep helps determine the optimal momentto wake, daylight has immediate alerting prop-erties. When light seeps through our eyelidsand touches our retinas, a signal is sent to adeep-brain region called the suprachiasmaticnucleus. This is the time, for many of us, thatour last dream dissolves, we open our eyes, andwe rejoin our real life.Or do we? Perhaps the most remark-able thing about REM sleep is thatit proves the brain can operateindependently of sensory input. Like an artistensconced in a secret studio, our mind appearsto experiment without inhibition, let loose onits own personal mission.When we’re awake, the brain is occupiedwith busy work—all those limbs to control, theconstant driving and shopping and texting andtalking. The money-earning, the child-rearing.But when we’re sleeping, and we commence``````Joe Diemand, 76,has spent the past20 years as a truckdriver, sometimesdriving all night. Suchwork, he says, leavesyou “so tired that youcan’t sleep.” The WorldHealth Organization hasdescribed night shiftwork as “probably car-cinogenic to humans.”76 NATIONAL GEOGRAPHIC