Sleep
During sleep, the brain uses significantly less energy than it does when awake. Research shows a 44% reduction in the cerebral metabolic rate of glucose and a 25% reduction in oxygen consumption during this state. This drop in energy demand allows the brain to restore its supply of adenosine triphosphate, the molecule used for short-term storage and transport of energy. In quiet waking, the brain is responsible for 20% of the body's total energy use, so this reduction has a noticeable effect on overall energy consumption.
Electrical activity seen on an electroencephalogram represents distinct brain waves that mark different stages of rest. Alpha waves appear when a person rests with closed eyes but remains fully conscious. Beta waves take over when attention focuses on a task or concentration heightens. These waves consist of the highest frequencies and lowest amplitude found in the cycle. Theta waves occur as a person transitions from wakefulness into Stage 1 of sleep. Delta waves emerge in Stages 3 and 4, marking the deepest phase of rest.
The internal circadian clock controls sleep timing through hormonal signals from a complex neurochemical system called Process C. The suprachiasmatic nucleus serves as the most important nexus for this process, located directly above the optic chiasm. This area shows conspicuous oscillation activity that intensifies during subjective day and drops to almost nothing during subjective night. The pineal gland releases melatonin at night under direct neural connection from the suprachiasmatic nucleus.
Exposure to even small amounts of light during the night can suppress melatonin secretion and increase body temperature. Blue light exerts the strongest effect, leading to concerns about screen use before bed. Modern humans often find themselves desynchronized from their internal clock due to work requirements, long-distance travel, and universal indoor lighting. A healthy young adult entrained to the sun falls asleep a few hours after sunset and wakes up a few hours after sunrise. Artificial light has substantially altered humanity's sleep patterns since its widespread adoption.
The sleeping brain removes metabolic end products at a faster rate than during an awake state by increasing cerebrospinal fluid flow. This mechanism appears to be the glymphatic system, which functions like the lymphatic system does for the body. Pulses of hormones create surges in blood flow that carry away metabolites while the brain rests. Anabolic hormones such as growth hormone are secreted preferentially during sleep to help repair and protect the brain.
Sleep also supports the formation of long-term memory through specific phases. Declarative memory improves more during early sleep dominated by slow-wave sleep. Procedural memory benefits during late sleep dominated by rapid eye movement sleep. Hippocampal replays of previously encoded neural patterns facilitate long-term consolidation. This dialogue between the hippocampus and neocortex occurs in parallel with sharp-wave ripples and thalamo-cortical spindles. Targeted memory reactivation experiments show that cued memories can strengthen recognition and lexical integration during rest.
Insomnia is the most common sleep problem, affecting 10, 15% of adults who report a chronic condition. Causes include psychological stress, poor sleep environments, inconsistent schedules, or excessive mental stimulation before bedtime. Sleep duration less than seven hours correlates with coronary heart disease and increased risk of death from that cause. Short sleep duration is associated with an increased risk of obesity ranging from 45% to 55%. Poor sleep is both a symptom and a cause of mental illness like major depressive disorder.
Up to 90 percent of adults with depression have sleep difficulties. Insomnia at the beginning of a study period indicates a more than twofold increased risk for developing major depressive disorder. Sleep disorders can increase the risk of psychosis and worsen the severity of psychotic episodes. African-Americans report experiencing short durations of sleep five times more often than whites, possibly due to social and environmental factors. Children in disadvantaged neighborhoods suffer higher rates of sleep apnea and poorer responses to treatment.
By the time infants reach age two, their brain size has reached 90 percent of an adult-sized brain. A majority of this growth occurs during the period of life with the highest rate of sleep. Newborns need up to 18 hours of sleep per day to develop and function properly. The National Sleep Foundation revised recommendations in early 2015 after a two-year study. Infants aged four to eleven months require 12 to 15 hours while toddlers need 11 to 14 hours.
School-age children between five and twelve years should get 9 to 11 hours of rest. Teenagers aged thirteen to seventeen require 8 to 10 hours each night. Adults from eighteen to sixty-four years need 7 to 9 hours. Older adults over sixty-five years benefit from 7 to 8 hours. Children who sleep through the night have higher cognitive attainments and easier temperaments. Infants who sleep longer at twelve months possess better vocabularies by twenty-six months.
Pre-industrial Europe practiced biphasic sleeping as the norm before artificial light became widespread. People without artificial light might go to sleep far sooner after sunset but wake several times throughout the night. Historian A. Roger Ekirch notes that segmented sleep began disappearing among urban upper classes in late 17th century Europe. By the 1920s, the idea of first and second sleep had receded entirely from social consciousness. Changes were attributed to street lighting, domestic lighting, and a surge in coffee houses making nighttime a legitimate time for activity.
In many nomadic or hunter-gatherer societies, people sleep on and off throughout day or night depending on events. Some societies display fragmented patterns where individuals sleep at all times for shorter periods. In some cultures, people sleep with at least one other person or even animals. Others rarely sleep with anyone except an intimate partner. Sleeping locations vary from ground level to platforms or beds shaped by climate, protection needs, and technology available.
Common questions
What percentage reduction in glucose metabolism occurs during sleep compared to wakefulness?
Research shows a 44% reduction in the cerebral metabolic rate of glucose and a 25% reduction in oxygen consumption during this state. This drop allows the brain to restore its supply of adenosine triphosphate, which is used for short-term storage and transport of energy.
How does the suprachiasmatic nucleus regulate melatonin release at night?
The suprachiasmatic nucleus serves as the most important nexus for Process C and releases signals that cause the pineal gland to release melatonin at night under direct neural connection. This area shows conspicuous oscillation activity that intensifies during subjective day and drops to almost nothing during subjective night.
Which specific memory types improve during slow-wave sleep versus rapid eye movement sleep?
Declarative memory improves more during early sleep dominated by slow-wave sleep while procedural memory benefits during late sleep dominated by rapid eye movement sleep. Hippocampal replays of previously encoded neural patterns facilitate long-term consolidation through dialogue between the hippocampus and neocortex.
What health risks are associated with sleeping less than seven hours per night?
Sleep duration less than seven hours correlates with coronary heart disease and increased risk of death from that cause. Short sleep duration is also associated with an increased risk of obesity ranging from 45% to 55%.
When did segmented sleep begin disappearing among urban upper classes in Europe?
Historian A. Roger Ekirch notes that segmented sleep began disappearing among urban upper classes in late 17th century Europe. By the 1920s, the idea of first and second sleep had receded entirely from social consciousness due to changes attributed to street lighting and coffee houses.