I had never given much thought to how length of day affected me until I spent a semester studying Russian in what was then Leningrad. Sure, a long winter night in Toronto can make you feel cozy and sleepy, and a long summer day can perk you up. But the kind of shift you see here at the 44th parallel is nothing compared with what you get at the 60th.
We arrived in January. During those first weeks, the only glimpse of daylight came during our lunch break, and even then the sun was pretty low in the sky. It rose after our classes began in the morning and set before they ended. We felt gloomy and slept a lot. Just 19 weeks later, however, our habitat had changed dramatically. By then, it was still light at midnight, and the sun set and rose during bedtime hours. But who was sleeping? We were euphoric.
In The Siesta and the Midnight Sun, journalist Jessa Gamble explores what's going on inside our bodies, and those of other animals, as we respond to time cues from the world around us. We mammals, it turns out, have a master clock in our brains, called the suprachiasmatic nucleus (SCN). It keeps time by using information on light levels that we collect through our eyes. And it lords it over other, more peripheral timepieces located in the heart, liver, lungs and other organs.
It's the SCN that helps control when we release melatonin, a hormone that triggers sleepiness. Even when snipped out of the brain and put into a Petri dish on a lab counter, Gamble writes, our master clock will pretty much keep to its 24-hour hour cycle, slowing itself around dusk and revving back up around dawn.
One of the most interesting issues raised in the book is about Western attitudes to sleep. Gamble points out that it may not have been the steam engine so much as the widespread use of the clock that caused the Industrial Revolution. Instead of natural schedules, people began adhering to imposed ones. Lunch now happened at lunchtime rather than when you were hungry. Work commenced with the workday rather than with first light. The idea has become so normal that we now expect even babies to sleep on strict schedules, as we have learned to do.
Gamble reminds us that it wasn't always the norm to expect an uninterrupted eight-hour block of sleep. Back when we relied less on artificial light, dusk and dawn marked the beginning and end of our bedtime. But in winter, if you were beyond the tropics, there was more darkness than a person needed for sleep. It was typical to enjoy a first sleep and a second – punctuated by a few hours of calm wakefulness in the middle of the night. (That is a diagnosable illness these days. For fun, I googled "nighttime wakefulness" and came up with "circadian rhythm sleep disorders" and a handy pharmaceutical solution.)
It's not just time of day that matters, but season too. Plants measure the length of each night to help them figure out when to bloom. From hamsters to sheep, animals use light cues to breed at a time of year that guarantees there will be food around when the young arrive.
Great migrations also rely on seasonal cues, Gamble writes. "For a Canada goose to prepare himself for spring migration, he needs considerable lead time to fatten up. He'll also molt his winter feathers and grow his gonads for mating season. Light-sensitive parts of his brain sense the sun's rays through the top of his skull and measure the length of each day. As he becomes aware of the increasing daylight hours – calculated using his circadian clock – his hormones respond with seasonally appropriate behaviour."
Gamble, who lives in Yellowknife, NWT, is young, but she has gravitas. She has worked with rats in the lab, learning about biological timekeeping first-hand. She has gone to all corners of the Earth and to great extremes of temperature to explore every angle of her topic. She writes accessibly about complicated ideas.
From how birds navigate across time zones to how caribou cope with perpetual daylight, from what happens to bedtime in the darkest place on the planet to when is the best time of day to compete in a sport, the book is a treasure trove of fact and narrative, well worth curling up with in the long dark days of winter.
Alison Motluk is a freelance science writer, who sleeps and wakes in Toronto