The Mental Consequences of Sleeplessness Sleep is also necessary for normal mental functioning. Chronic sleep deprivation increases levels of the stress hormone cortisol, which may damage or impair brain cells that are necessary for learning and memory (Minkel et al., 2014). Also, new brain cells may either fail to develop or may mature abnormally (Guzman-Marin et al., 2005). Perhaps in part because of such damage, after the loss of even a single night's sleep, mental flexibility, attention, and creativity all suffer. After several days of staying awake, people may even begin to have hallucinations and delusions (Dement, 1978).

Of course, sleep deprivation rarely reaches that point, but people do frequently suffer from milder sleep problems. According to the National Sleep Foundation, about 10 percent of adults are plagued by difficulty in falling or staying asleep. The causes of their insomnia include worry and anxiety, psychological problems, physical problems such as arthritis, and irregular or overly demanding work and study schedules. In addition, many drugs interfere with the normal progression of sleep stages—not just the ones containing caffeine, but also alcohol and some tranquilizers. The result can be grogginess and lethargy the next day.

Another cause of daytime sleepiness is sleep apnea, a disorder in which breathing periodically stops for a few moments, causing the person to choke and gasp. Breathing may cease hundreds of times a night, often without the person knowing it. Sleep apnea is seen most often in older males and overweight people but also occurs in others. It has several causes, from blockage of air passages to failure of the brain to control respiration correctly. Over time, it can cause high blood pressure and irregular heartbeat; it may gradually erode a person's health and is associated with a shortened life expectancy (Young et al., 2008).

With narcolepsy, an even more serious disorder that often develops in the teenage years, an individual is subject to irresistible and unpredictable daytime attacks of sleepiness lasting from 5 to 30 minutes. The cause is not well understood, but the disorder has been associated with reduced amounts of a particular brain protein, possibly brought on by an autoimmune problem, a viral infection, or genetic abnormalities (Baumann et al., 2014; Kornum, Faraco, & Mignot, 2011; Mieda et al., 2004). When the person lapses into sleep, he or she is likely to fall immediately into the REM stage. Some people with narcolepsy experience an unusual symptom called cataplexy, which brings on the paralysis of REM sleep although they are still awake; as a result, they may suddenly drop to the floor. Cataplexy is often triggered by laughing excitedly, but it can sometimes be induced by telling a joke or even having an orgasm (Overeem et al., 2011). You can learn more about these conditions by watching the video Sleep Disorders.

Other disorders also disrupt sleep, including some that cause odd or dangerous behavior. In REM behavior disorder, the muscle paralysis associated with REM sleep does not occur, and the sleeper (usually an older male) becomes physically active, often acting out a dream without any awareness of what he is doing (Randall, 2012; Schenck & Mahowald, 2002). If he is dreaming about football, he may try to “tackle” a piece of furniture; if he is dreaming about a kitten, he may try to pet it. Other people may consider this disorder amusing, but it is no joke; sufferers may hurt themselves or others, and they have an increased risk of later developing Parkinson's disease and dementia (Mariotti et al., 2015).

However, the most common cause of daytime sleepiness is the most obvious one: simply not getting enough sleep. Some people do fine on relatively few hours, but most adults need more than 6 hours for optimal performance, and many adolescents need 10. In the United States, drowsiness is involved in 100,000 vehicle accidents a year, causing 1,500 road deaths and 71,000 injuries. Sleep deprivation also leads to accidents and errors in the workplace, a concern especially for first-year doctors doing their medical residency. Although federal law limits work hours for airline pilots, truck drivers, and nuclear-plant operators, in many states, medical residents often still work 24- to 30-hour shifts (Landrigan et al., 2008).

Don't doze off as we tell you this, but lack of sleep has also been linked to reduced alertness in school and lower grades. In 1997, a high school in Minneapolis changed its start time from 7:20 a.m. to 8:30 a.m. Teachers watched in surprise as students became more alert and—according to their parents—“easier to live with” (Wahlstrom, 2010). Since then, many other school districts in the United States and other countries have followed suit by starting school later in the morning (Vedaa et al., 2012). Children and teenagers who start school later sleep more, have improved mood, are able to pay more attention in class, and get better test scores; teenage drivers even have fewer car accidents (Fallone et al., 2005; Vorona et al., 2011).

The Mental Benefits of Sleep Just as sleepiness can interfere with good mental functioning, a good night's sleep can promote it, and not just because you are well rested. In a classic study conducted nearly a century ago, students who slept for 8 hours after learning lists of nonsense syllables retained them better than students who went about their usual business (Jenkins & Dallenbach, 1924). For years, researchers attributed this result to the lack of new information coming into the brain during sleep, information that could interfere with already-established memories. Today, however, most believe that sleep is a crucial time for consolidation, in which synaptic changes associated with recently stored memories become durable and stable.

One theory is that while we are sleeping, the neurons that were activated during the original experience are reactivated, promoting the transfer of memories from temporary storage in the hippocampus to long-term storage in the cortex and thus making those changes more permanent (Born & Wilhelm, 2012). During sleep, consolidation seems to target important information that we know we might need later. When researchers had people learn new information and then let them sleep, those who were told before sleeping that they would later be taking a memory test did better on it than those who did not know about the upcoming test (Wilhelm et al., 2011). Sleep seems to strengthen many kinds of memories, including the recollection of events, locations, facts, and emotional experiences, especially negative ones (see Figure5.4).

Figure5.4

Sleep and Memory Consolidation

Memory consolidation is most closely associated with the slow waves of neural activity that occur during Stage N-REM 3 of sleep. In one study, while scientists taught people the locations of several matching pairs of cards (a memory game), they also presented the scent of roses. Later, some people were exposed to the smell of roses again during slow-wave sleep, REM sleep, or when they were awake. Exposure to the smell during slow-wave sleep, but not REM or wakefulness, improved people's memories for the card locations (Rasch et al., 2007). But REM sleep does seem to be related to some improvements in learning and memory (Mednick et al., 2011). When people or animals learned a perceptual task and were allowed to get normal REM sleep, their memory for the task was better the next day, even when they had been awakened during non-REM periods. When they were deprived of REM sleep, however, their memories were impaired (Karni et al., 1994). Thus, both periods of sleep seem to be important for consolidation, and scientists are now trying to determine their respective roles (Born & Wilhelm, 2012). To find out more about how sleep deprivation affects cognitive functioning, watch the video Sleep, Memory, and Learning.

If sleep enhances memory, perhaps it also enhances problem solving, which relies on information stored in memory. To find out, German researchers gave volunteers a math test that required them to use two mathematical rules to generate one string of numbers from another and to deduce the final digit in the new sequence as quickly as possible. The volunteers were not told about a hidden shortcut that would enable them to calculate the final digit almost immediately. One group was trained in the evening and then got to snooze for 8 hours before returning to the problem. Another group was also trained in the evening but then stayed awake for 8 hours before coming back to the problem. A third group was trained in the morning and stayed awake all day, as they normally would, before taking the test. Those people who got the nighttime sleep were nearly three times likelier to discover the hidden shortcut as those in the other two groups (Wagner et al., 2004).

Sleep, then, seems essential in memory and problem solving. The underlying biology appears to involve not only the formation of new synaptic connections in the brain but also the weakening of connections that are no longer needed (Donlea, Ramanan, & Shaw, 2009; Gilestro, Tononi, & Cirelli, 2009; Michel & Lyons, 2014). In other words, we sleep to remember, but we also sleep to forget, so that the brain will have space and energy for new learning. Remember that the next time you are tempted to pull an all-nighter. Even a quick nap may help your mental functioning and increase your ability to put together separately learned facts in new ways (Lau, Alger, & Fishbein, 2011; Mednick et al., 2002). Sleep on it.