The Dark Side of the Brain : NIGHT: Night Life, Night Language, Sleep and Dreams, <i> By A. Alvarez (W.W. Norton: $23; 277 pp.)</i>

Over the course of the last century, a new nation on the earth has been discovered and explored, a place that for all the previous centuries had been shrouded in darkness and mystery--the nation of the night.

Thomas Edison was the Columbus of this new nation, lighting the way with an electric bulb that finally allowed people to stay up later than the sun. The night was soon purged of many of its old fears, and overrun by waves of new settlements--factories, restaurants, convenience stores, Wall Street trading floors, the Home Shopping Network. The average night’s sleep shrank (from 9 1/2 hours in the 19th Century to barely more than seven hours today) and the after-dark population soared (to 10 million, the number of Americans awake at 3 a.m. each night now).

While the external night was growing brighter and busier, a different set of explorers were mapping the internal night--the world that comes alive in the minds of sleeping dreamers. Psychologists, neuroscientists and sleep researchers have drawn a clearer picture of just what happens to us when we get into bed and close our eyes. They have shown that sleep, like the night itself, is not the dark oblivion it appears on the surface, but is instead a complex state of teeming activity.

This dual revolution in our experience of the night--our understanding of its interior architecture, our behavior in its exterior landscape--has been swift and silent, largely unexamined by the lay public, and it has provided A. Alvarez with a luminous topic for his latest book. “Night” is a quirky, elegantly written tour of this newly discovered territory--less a travelogue in form than a meditation on the themes and ideas that animate the hours of darkness.

It is a big territory, easy to get lost in, but Alvarez is one of those intrepid British writers well-equipped for the trek. Blissfully resisting narrow categorization, accomplished in many genres--poetry, criticism, fiction, nonfiction--he ventures into the night as an inquisitive man of letters, equally comfortable quoting Milton, interpreting Freud or riding in the back seat of a New York City police cruiser.

Alvarez starts his journey, logically enough, with a brief history of artificial light, the tool that made possible the conquest of the night. “Illumination,” as he reminds us, “is one of the few 20th-Century experiments that hasn’t failed.” From there, torch in hand, he follows a circuitous route through the darkness, guided by an itinerary that has the loose, free-associational quality of a dream.

“Night contains whatever you care to put into it,” he writes, and what he puts into it is just about anything that piques his curiosity, which is a lot.

West Indians, Alvarez informs us, used to stick fireflies to their big toes with gum, trying to see snakes in the path at night. When Michelangelo was painting the ceiling of the Sistine Chapel, he fashioned a crude work lamp for himself, a headband that held a stub of candle. The word curfew comes from the Norman-French “covre-le-feu”--douse the fire. In the late 17th Century, the city of Paris burned 1,625 pounds of candles in its street lamps each night. Platypuses and spiny anteaters don’t dream, but almost every other mammal does. Humans spend between 90 minutes and two hours each night--between five and six years over the course of a lifetime--lost in their dreams.

These strange and wonderful dreams--and the deep forest of sleep that shelters and sustains them--are what interest Alvarez most, and what consume the bulk of the book. From the ancient Greeks to the modern neurologists, he deftly surveys the evolving theories about how and why we sleep and dream, writing in clear and lucid prose that manages to convey the science without losing the poetry.

Quoting prodigiously, and citing examples as diverse as Coleridge and the Marx Brothers, he shows how art and literature have anticipated science, sketching the rough outlines that laboratory research would later fill in. He offers striking evidence of how active our sleeping brains really are (even in the deepest sleep, neuronal activity declines just 5% to 10%), and how accomplished our dreams can be in solving problems (the final, troublesome piece of Elias Howe’s sewing machine appeared to him in a dream). He reminds us of how little we knew about sleep and dreams until recently (the discovery of rapid eye movement (REM) sleep coincided roughly with the beginnings of space exploration, as he observes in a typically insightful aside), and how much we still have to learn.

“(W)hat goes on inside the head,” he writes, “is as strange and baffling as what goes on in remote corners of remote galaxies.”

Like the night itself, the book drags at times, slowed by spots where the author lingers longer than the reader might have wished. Lacking a straight and obvious path through his material, Alvarez holds his narrative together with a single, continuing character--himself. His use of the first-person is judicious, neither preaching nor preening, and more successful in some places than others. The strategy works well early in the book--when he recounts his childhood fear of the dark, or has himself wired up in a sleep lab. It feels forced and tacked on later--as when he rides along with cops in London and Manhattan.

Despite his occasional meandering, though, Alvarez does lead you to a new understanding of the night. You will no longer look out into the darkness and see only a yawning void. You will look deeper into the interior night, too. You have five separate dreams each night, Alvarez reports, maybe one of which you’ll be lucky enough to remember. After reading this book, you might start keeping a notebook on the night stand, trying to catch the other four.