Somnolence (the scientific term for sleep which literally means “drowsiness”) is derived from the word “somnus” which is Latin for “sleep”. Sleep refers to a naturally recurring state of mind and body characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of being comatose. Sleep occurs in repeating periods, in which the body alternates between two highly distinct modes known as non-REM and REM sleep. Although REM stands for “rapid eye movement”, this mode of sleep has many other aspects, including virtual paralysis of the body. A well-known feature of sleep is the dream, an experience typically recounted in narrative form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy. During sleep, most of the body’s systems are in an anabolic state, helping to restore the immune, nervous, skeletal, and muscular systems; these are vital processes that maintain mood, memory, and cognitive performance and plays a large role in the function of the endocrine and immune systems. The internal circadian clock promotes sleep daily at night. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. Moreover the advent of artificial light has substantially altered sleep timings across the world.
Awakening refers to the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken from slow-wave sleep, soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep episode. Awakening involves heightened electrical activation in the brain, beginning with the thalamus and spreading throughout the cortex. Many humans awaken due to alarm clocks. Some people, however, can reliably wake themselves up at a specific time with no need for an alarm. Many sleep quite differently on workdays versus holidays, a pattern which can lead to chronic circadian desynchronization. Many people regularly look at television and other screens before going to bed, a factor which may exacerbate this mass circadian disruption. Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying sleep inertia.
This internal circadian clock is profoundly influenced by changes in light, since these are the body’s primary evidences regarding time. Exposure to even small amounts of light during the night can suppress melatonin secretion, increase body temperature, and increase cognitive ability. Short pulses of light, at the right moment in the circadian cycle, can significantly reset the internal clock. Blue light, in particular, exerts the strongest effect leading to concerns that electronic media use before bed may interfere with sleep. Today’s humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially night shifts), long-distance travel, and the influence of widespread indoor lighting. Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely they can have difficulty waking up in the trough of the cycle.
Scientists have demonstrated in various ways that sleep enhances memory. Procedural memory benefits from late, REM-rich sleep, whereas declarative memory benefits from early, slow wave-rich sleep. Wound healing is known to be affected by sleep. It has been shown that sleep deprivation affects the immune system. Sleep loss impairs immune function and immune challenge alters sleep, and it has been suggested that sleep increases white blood cell count. Sleep deprivation increases cancer growth and dampens the immune system’s ability to control cancers. Sleep also combats the accumulation of free radicals in the brain, by increasing the efficiency of endogenous antioxidant mechanisms. Sleep hygiene is the recommended behavioral and environmental practice that is intended to promote better quality sleep. While the term sleep hygiene was first introduced in 1939 by Nathaniel Kleitman, a book published in 1977 by psychologist Peter Hauri introduced the concept within the context of modern sleep medicine. In his book Hauri outlined a list of behavioral rules intended to promote improved sleep. Similar concepts are credited to Paolo Mantegazza who published a related original book in 1864. The 1990 publication of the International Classification of Sleep Disorders (ICSD) introduced the diagnostic category Inadequate Sleep Hygiene. Inadequate sleep hygiene was a subclassification of Chronic Insomnia Disorder in the ICSD-II published in 2005; it was removed from the 2014 ICSD-III along with two other classifications, because “they were not felt to be reliably reproducible in clinical practice”. Specific sleep hygiene recommendations have changed over time. Sleep hygiene is a central component of cognitive behavioral therapy for insomnia. Sleep hygiene recommendations have been shown to reduce or eliminate the symptoms of insomnia. Specific sleep disorders may require other or additional treatment approaches, and continuing difficulties with sleep may require additional assistance from healthcare providers. College students are at risk of engaging in poor sleep hygiene and also of being unaware of the resulting effects of sleep deprivation. Because of irregular weekly schedules and the campus environment, college students may be likely to have variable sleep-wake schedules across the week, take naps, drink caffeine or alcohol near bedtime, and sleep in disruptive sleeping environments. Similarly, shift workers have difficulty maintaining a healthy sleep-wake schedule due to night or irregular work hours. Shift workers need to be strategic about napping and drinking caffeine, as these practices may be necessary for work productivity and safety, but should be timed carefully. Because shift workers may need to sleep while other individuals are awake, additional sleeping environment changes should include reducing disturbances by turning off phones and posting signs on bedroom doors to inform others when they are sleeping. Due to symptoms of low mood and energy, individuals with depression may be likely to have behaviors that are counter to good sleep hygiene, such as taking naps during the day, consuming alcohol near bedtime, and consuming large amounts of caffeine during the day. In addition to sleep hygiene education, bright light therapy can be a useful treatment for individuals with depression. Not only can morning bright light therapy help establish a better sleep-wake schedule, but it also has been shown to be effective for treating depression directly, especially when related to seasonal affective disorder.
Individuals with breathing difficulties due to asthma or allergies may experience additional barriers to quality sleep that can be addressed by specific variations of sleep hygiene recommendations. Difficulty with breathing can cause disruptions to sleep, reducing the ability to stay asleep and to achieve restful sleep. For individuals with allergies or asthma, additional considerations must be given to potential triggers in the bedroom environment. Medications that might improve ability to breathe while sleeping may also impair sleep in other ways, so there must be careful management of decongestants, asthma controllers, and antihistamines.
Exercise is an activity that can facilitate or inhibit sleep quality; people who exercise experience better quality of sleep than those who do not, but exercising too late in the day can be activating and delay falling asleep. Increasing exposure to bright and natural light during the daytime and avoiding bright light in the hours before bedtime may help promote a sleep-wake schedule aligned with nature’s daily light-dark cycle.
Activities that reduce cognitive activity promote falling asleep, so engaging in relaxing activities before bedtime is recommended. Conversely, continuing important work activities or planning shortly before bedtime or once in bed has been shown to delay falling asleep. Similarly, good sleep hygiene involves minimizing time spent thinking about worries or anything emotionally upsetting shortly before bedtime. Trying purposefully to fall asleep may induce frustration that further prevents falling asleep, so in such situations a person may be advised to get out of bed and try something else for a brief amount of time.
Sleep deprivation can be used as a means of interrogation, which has resulted in court trials over whether or not the technique is a form of torture. Sleep deprivation tends to cause slower brain waves in the frontal cortex, shortened attention span, higher anxiety, impaired memory, and a grouchy mood. Conversely, a well-rested organism tends to have improved memory and mood. For adults, getting less than 7–8 hours of sleep is associated with a number of physical and mental health deficits, and therefore a top sleep hygiene recommendation is allowing enough time for sleep. Clinicians will frequently advise that these hours of sleep are obtained at night instead of through napping, because while naps can be helpful after sleep deprivation, under normal conditions naps may be detrimental to night-time sleep. Negative effects of napping on sleep and performance have been found to depend on duration and timing, with shorter midday naps being the least disruptive.