Why did you choose go to sleep last night at the particular time you did?
Maybe you were just plain tired. But, chances are, there were other factors involved in that decision, as well. Where you hoping to get a certain number of hours of rest before you had to get up and go to work? Maybe it just felt like time to crawl into bed, because your friends and family were, too. If you stayed up later, would you feel like you were doing something wrong? Do your sleep patterns change when you've spent time in another country?
Sleep, and the physical cycles that drive it, aren't just about biology. The patterns and expectations surrounding sleep have varied greatly throughout human history and from place to place. Sleep is cultural. If you want to understand the science of sleep, you have to learn both biochemistry and anthropology.
That's the message at the heart of Jessa Gamble's The Siesta and the Midnight Sun. This is a book about how circadian rhythms work. But it's also a book about how the invention of the clock and the long arm of Western colonialism changed the way human beings relate to the world around them in a really fundamental way.
I'll be honest. That perspective took me off-guard. I dove into this book expecting to learn some cool biology. And I did. Gamble spends the first third of the book talking about how seasonal and daily rhythms manifest in animals and plants … and how we see traces of those rhythms in humans. You'll discover that in some species, like sheep, hormone levels drop and the testes actually retreat into the male's body for much of the year, preventing breeding at times when food is scare. And you'll find that male humans still carry echoes of this: Their reproductive hormones peak at the beginning of summer and drop off at the end of summer, taking condom sales and STD diagnoses along with them.
I also expected to learn some practical advice about what we should do to sleep in a more healthy way. That's in the book, too. Gamble actually recommends a couple of cool smart phone applications. Jet Lag Fighter helps you prepare for a cross-country or trans-oceanic flight by slowly acclimatizing you to different sleep patterns over a number of days leading up to your trip. Sleep Cycle alarm clock asks you to sleep with your head on your phone. It then uses the phone's accelerometer to monitor your movement and determine where you are in your sleep cycles. That way, you can make sure that your alarm goes off when you're already at a light stage of sleep, which leaves you feeling more rested.
(There's also a really interesting section where Gamble discusses the perils of a fad "natural" health trend that recommended people change their sleep pattern to take multiple short naps throughout the day—and never really sleep for hours at all.)
But The Siesta and the Midnight Sun is really at its most intriguing (and Gamble's writing is at its best) when the book moves past the biological basics and into the deeper story of how and why humans choose to go against their circadian rhythms, and how those changes affect us.
This involves a lot of cultural relativism—sleep has always meant something different to people in the Arctic compared to people in the tropics. It involves some critique of how cultures dominate and change each other—before the 9-5 work day, Arctic cultures were able to adapt their lives to the demands of extreme seasons and the midnight sun in a much more comfortable way. And it also involves issues of class. It is impossible right now to read about what happens to a human body subjected to shift work, particularly swing shift work, without thinking of it in the context of the 99%. As a well-off, self-employed person, I have the privilege of adjusting my sleep schedules to match what makes me feel healthy. Meanwhile, someone who needs to work the late shift accepts an increased risk of cancer and what amounts to constant, life-long jet lag in exchange for a paycheck.
Ultimately, The Siesta and the Midnight Sun is a lot like the biological processes it documents and deconstructs—utterly fascinating, but not always comfortable. And that's a good thing.