One of the consequences of waking up suddenly, and too early, is a phenomenon called sleep inertia. First given a name in 1976, sleep inertia refers to that period between waking and being fully awake when you feel groggy. The more abruptly you are awakened, the more severe the sleep inertia. While we may feel that we wake up quickly enough, transitioning easily between sleep mode and awake mode, the process is in reality far more gradual. Our brain-stem arousal systems (the parts of the brain responsible for basic physiological functioning) are activated almost instantly. But our cortical regions, especially the prefrontal cortex (the part of the brain involved in decision-making and self-control), take longer to come on board.

In those early waking minutes, our memory, reaction time, ability to perform basic mathematical tasks, and alertness and attention all suffer. Even simple tasks, like finding and turning on the light switch, become far more complicated. As a result, our decisions are neither rational nor optimal. In fact, according to Kenneth Wright, a neuroscientist and chronobiology expert, “Cognition is best several hours prior to habitual sleep time, and worst near habitual wake time.” In the grip of sleep inertia, we may well do something we know we shouldn’t. Whether or not to hit the snooze button is just about the first decision we make. Little wonder that it’s not always the optimal one.

Other research has found that sleep inertia can last two hours or longer. In one study that monitored people for three days in a row, the sleep researchers Charles Czeisler and Megan Jewett and their colleagues at Harvard Medical School found that sleep inertia took anywhere from two to four hours to disappear completely. While the participants said they felt awake after two-thirds of an hour, their cognitive faculties didn’t entirely catch up for several hours. Eating breakfast, showering, or turning on all the lights for maximum morning brightness didn’t mitigate the results. No matter what, our brains take far longer than we might expect to get up to speed.

When we do wake up naturally, as on a relaxed weekend morning, we do so based mainly on two factors: the amount of external light and the setting of our internal alarm clock—our circadian rhythm. The internal clock isn’t perfectly correlated with the external one, and so every day, we use outside time cues, called zeitgebers, to make fine adjustments that mimic the changes in light and dark that take place throughout the year.

The difference between one’s actual, socially mandated wake-up time and one’s natural, biologically optimal wake-up time is something that Till Roenneberg, a professor of chronobiology at Ludwig-Maximilians University in Munich, calls “social jetlag.” It’s a measurement not of sleep duration but of sleep timing: Are we sleeping in the windows of time that are best for our bodies? According to Roenneberg’s most recent estimates, based on a database of more than sixty-five thousand people, approximately a third of the population suffers from extreme social jetlag—an average difference of over two hours between their natural waking time and their socially obligated one. Sixty-nine per cent suffer from a milder form, of at least one hour.

Roenneberg and the psychologist Marc Wittmann have found that the chronic mismatch between biological and social sleep time comes at a high cost: alcohol, cigarette, and caffeine use increase—and each hour of social jetlag correlates with a roughly thirty-three per cent greater chance of obesity. “The practice of going to sleep and waking up at ‘unnatural’ times,” Roenneberg says, “could be the most prevalent high-risk behaviour in modern society.” According to Roenneberg, poor sleep timing stresses our system so much that it is one of the reasons that night-shift workers often suffer higher-than-normal rates of cancer, potentially fatal heart conditions, andother chronic disease, like metabolic syndrome and diabetes. Another study, published earlier this year and focussing on medical-school performance, found that sleep timing, more than length or quality, affected how well students performed in class and on their preclinical board exams. It didn’t really matter how long they had slept or whether they saw themselves as morning people or not; what made a difference was when they actually went to bed—and when they woke up. It’s bad to sleep too little; it’s also bad, and maybe even worse, to wake up when it’s dark.

Fortunately, the effects of sleep inertia and social jetlag seem to be reversible. When Wrightasked a group of young adults to embark on a weeklong camping trip, he discovered a striking pattern: before the week was out, the negative sleep patterns that he’d previously observed disappeared. In the days leading up to the trip, he had noted that the subjects’ bodies would begin releasing the sleep hormone melatonin about two hours prior to sleep, around 10:30 P.M.A decrease in the hormone, on the other hand, took place after wake-up, around 8 A.M. After the camping trip, those patterns had changed significantly. Now the melatonin levels increased around sunset—and decreased just after sunrise, an average of fifty minutes before wake-up time. In other words, not only did the time outside, in the absence of artificial light and alarm clocks, make it easier for people to fall asleep, it made it easier for them to wake up: the subjects’ sleep rhythms would start preparing for wake-up just after sunrise, so that by the time they got up, they were far more awake than they would have otherwise been. The sleep inertia was largely gone.

Wright concluded that much of our early morning grogginess is a result of displaced melatonin—of the fact that, under current social-jetlag conditions, the hormone typically dissipates two hours after waking, as opposed to while we’re still asleep. If we could just synchronize our sleep more closely with natural light patterns, it would become far easier to wake up. It wouldn’t be unprecedented. In the early nineteenth century, the United States had a hundred and forty-four separate time zones. Cities set their own local time, typically so that noon would correspond to the moment the sun reached its apex in the sky; when it was noon in Manhattan, it was five till in Philadelphia. But on November 18, 1883, the country settled on four standard time zones; railroads and interstate commerce had made the prior arrangement impractical. By 1884, the entire globe would be divided into twenty-four time zones. Reverting to hyperlocal time zones might seem like it could lead to a terrible loss of productivity. But who knows what could happen if people started work without a two-hour lag, during which their cognitive abilities are only shadows of their full selves?

Theodore Roethke had the right idea when he wrote his famous line “I wake to sleep, and take my waking slow.” We do wake to a sleep of sorts: a state of not-quite-alertness, more akin to a sleepwalker’s unconscious autopilot than the vigilance and care we’d most like to associate with our own thinking. And taking our waking slow, without the jar of an alarm and with the rhythms of light and biology, may be our best defense against the thoughtlessness of a sleep-addled brain, a way to insure that, when we do wake fully, we are making the most of what our minds have to offer.

Maria Konnikova is the author of “Mastermind: How to Think Like Sherlock Holmes.”

Photograph: Keystone-France/Gamma-Keystone/Getty.