If you want to ace your exams, you’ll need to study hard. But, says Fred Davis, a biology professor with expertise in circadian rhythms, you’ll also need to put down the books and catch a little shuteye. Here’s why.

Gawker captured the value of catching a little shuteye when you have an exam on the horizon with this headline: “Want to memorize something? Take a nap.” Scientifically speaking, how does sleep help us retain information?

Experience tells us that sleep has a purpose; when we don’t get enough we feel and perform poorly and we develop an overwhelming drive to get sleep. Despite this, the functions of sleep and the reasons it evolved in so many animals are not fully understood. Three favored hypotheses are 1) a state in which cells restore themselves; 2) a way to conserve energy; and 3) a state that promotes learning and memory. The brain uses a lot of energy, 10 times more than predicted based on weight alone. The formation of long-term memories, to remember something for more than a few minutes or hours, requires energy for the synthesis of new molecules and for stronger connections among nerve cells. The activation of the brain by sensations during wakefulness also requires energy and it is a challenge for the brain to sort out which experiences should be transferred to long-term memory. Sleep provides a state with reduced sensations when the brain has the energy to select and make long-term memories. Experiments in humans and in laboratory animals have shown that learning and memory are disrupted when subsequent sleep is prevented and that sleep allowed soon after learning improves memory.

 

Studies show that pulling all-nighters to cram for exams can backfire, that sacrificing sleep to study is actually associated with performing more poorly on tests. What are the telltale signs that your body is craving sleep, that you should close your textbook and crawl under the covers?

If you have been sleeping less than seven hours a night, you are likely already in a state of sleep deprivation even before you start an all-nighter. If you sleep much longer than seven hours on some days, this is a sign that you are not getting enough sleep over the long term. Similarly, if you fall asleep at anytime during the day, such as in class, you are sleep deprived. Sleep deprivation can affect learning, reacting, solving problems, decision making, memory, and emotional control. If you read the same passage over and over again without knowing what you read, you might as well stop. Not surprisingly, the performance of someone who as been awake for 24 hours (or slept six hours per night for two weeks) is similar to that of someone with a blood alcohol level of 0.1 percent. If your head jerks and you realize that your eyes were closed, you experienced a microsleep, a sure sign of overwhelming fatigue. Microsleep episodes can even occur in localized regions of the brain, disabling those areas even though you feel you are awake. The best practice, of course, is to avoid the need for an all-nighter in the first place.

In The New Yorker’s three-part series on sleep, journalist Maria Konnikova laments, “we are very bad at knowing how much sleep is enough.” In your view, how many hours should the typical college student be sleeping per night? What are the unintended consequences of inadequate sleep?

How much sleep we need is a tricky question, in part because we don’t fully understand what sleep is for. A recent review of more than 5,000 studies concluded that seven or more hours of sleep per night on a regular basis is required for optimal health. Studies on the health effects of insufficient sleep commonly allow five hours of sleep a night and see effects in only a few days. Our brains need sleep to function optimally—and safely—but too little sleep also affects our health. Sleep deprivation affects inflammation, immune function, and metabolism, contributing to cardiovascular disease, obesity, and diabetes. If you eat when your body “knows” it should be asleep, according to your 24-hour biological clock, the food is metabolized differently. At different “body times” the same amount of food may or may not cause weight gain. Disrupted sleep or short sleep is commonly associated with anxiety and psychiatric disorders such as depression and schizophrenia. Cause and effect are difficult to disentangle, but normalizing sleep timing and duration are important components of treatment. Awake at night also means exposure to light when our body does not expect it, with resulting effects on our physiology. While duration is the most commonly measured dimension of normal sleep, sleep quality and regularity are also important—points you should discuss with your roommate.