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Summary

Circadian rhythm: humans evolved with circadian rhythm; owing to the gravity of the moon, every passing day becomes slightly longer; a leaf kept in phase (ie, exposed to day and night) stays fresh longer than one kept in constant light or darkness; cyanobacteria are the first eukaryotes known to have circadian rhythm; in human societies, the agricultural, industrial, and digital revolutions took place within the last ≈500 generations; each cell has a molecular clock at the nuclear level, and organs run on a circadian clock; the body runs on a 24.3-hr cycle; zeitgeber — anything that readjusts the circadian clock, the most potent being light, which enters through the suprachiasmatic nucleus; regulating light exposure, food, and exercise is important for health

Regulation of light: core body temperature minimum (CBTM) is achieved between 2 and 3 hr before routine waking time; giving light stimulus before CBTM is reached pushes the body clock forward, and giving it after reaching CBTM pushes the clock back; rhythms change over the span of a lifetime (in childhood sleep patterns conform to, eg, time zone to the east, in adolescence to time zone to the west, in adulthood to time zone to the east again)

Social jetlag: this occurs in adolescence and young adulthood when, as a result of staying up late at night on weekends, the body clock is pushed toward a time zone to the west; this results in difficulty falling asleep on workdays because the melatonin spike happens later in the evening; this can be corrected by providing light soon after reaching CBTM; providing light in the evenings pushes the clock to a time zone to the west; people transitioning from evening to day shifts benefit from morning light and should not wear sunglasses; humans evolved with bright morning light and dim evening light, but now live in perpetual twilight by wearing sunglasses or staying indoors during the day and having prolonged exposure to light at night; people working evening or night shifts should wear sunglasses after work (ie, in the morning); consistently working day shifts is ideal, night shifts less so, and alternating day and night shifts is the worst; individuals working night shifts often stay awake on their days off to do other things; ability to handle shift work is determined genetically; individuals who find shift work difficult should try to find less taxing schedules

Advanced sleep phase: the body clock is advanced relative to actual time, as seen in many older people who go to bed and wake up unusually early

Use of lighting: advanced sleep phase can be corrected by exposure to light (eg, watching television) in the evenings; social jetlag can be corrected by taking melatonin in the evening and light exposure in the morning; smart lighting can mimic natural lighting conditions; seasonal affective disorder lights reduce melatonin; though much dimmer than sunlight, indoor lighting can disrupt circadian rhythm in the evening; night-shift workers may benefit from using blackout curtains and sunglasses during the day; the night-shift mode in many electronic devices turns the screen orange and mellows it in dim light; people working night shifts should not use these while at work

Regulation of diet: circadian rhythm is related to weight management, nonalcoholic steatohepatitis, and insulin sensitivity (which correlates with aging); each calorie eaten after sunset has a higher effect than if eaten at noon, since the gut has melatonin receptors; eating at the same time daily can protect from cardiometabolic factors; it is advisable to have ≥12 hr of fasting daily (ie, between dinner and breakfast) and to maintain a consistent pattern; the human microbiome has its own circadian rhythm, transplanting stool from a jetlagged person to a germ-free rat causes the rat to gain weight; plasma lipids are also affected by circadian rhythm; the microbiome is passed from mothers to babies during birth and the first few hours of life; many neurotransmitters are made in the gut and travel to the brain; eating a plant-based, varied diet with more fermented foods and prebiotics is beneficial; antibiotics should be avoided unless absolutely necessary; in animal models, a higher-fiber diet correlates with better sleep

Melatonin and sleep requirements: 0.5 mg of melatonin, taken at approximately 6 PM, shifts the body clock backward; hypnotic (3 mg) doses should not be taken frequently as it loses potency; daily sleep requirement decreases with age and varies from person to person

Sleep-wake cycles: delta (deep) sleep — occurs in the early part of the night; on electroencephalography, it is seen as large waves; memory is consolidated during delta sleep; rapid eye movement (REM) or dream sleep — after a traumatic experience, REM sleep erases the impact of the event while retaining the memory; posttraumatic nightmares are caused by the brain reliving memories to erase their impact, but it is unable to do this; wearable devices — use of these to identify stages of sleep have not yet been validated; glymphatic network — misfolded proteins act as niduses of infection (eg, Alzheimer disease), but the blood-brain barrier usually traps them in the brain; the glymphatic network opens in the early hours of deep sleep and flushes detritus from the brain; deeper sleep can be achieved by reducing caffeine and alcohol intake and maintaining a regular schedule

Insomnia: drugs for insomnia are often ineffective and unsafe; cognitive behavioral therapy (CBT) provides longer term benefits, eliminates the risk for adverse effects, and is associated with lower inflammatory markers; drugs are appropriate for patients dealing with trauma or crises; online resources and CBT are beneficial; meditation improves sleep; thoughts should not be avoided, but observed without self-judgment; 3P model of insomnia — in earlier phases of evolution, sleep was a time during which one could, eg, be eaten; there are 2 neuronal centers for sleep compared to 10 to keep one awake; trauma in early life predisposes one to insomnia; events such as menopause, trauma, divorce, and illness precipitate it, and if left untreated, the condition is perpetuated

Sleep apnea: in humans, the tongue falls back and closes off the airway as a result of being able to speak; sleep apnea is perceived as a disease of obese, middle-aged men, but can occur in young women too; women tend not to be heavy snorers; in women with fatigue, if anemia is ruled out, sleep apnea may be the cause; sleep studies can be performed with take-home devices; sleep apnea can cause atrial fibrillation; alcohol intake worsens symptoms

Iron: the hippocampus needs iron; in people with restless legs syndrome, even if ferritin levels are normal, iron may not enter the brain; with menstruation or in people with fatigue, ferritin levels of ≈50 μg/L should be targeted; fasting iron and transferrin levels should be checked; for patients with restless legs syndrome, there is a low threshold for giving intravenous iron; if symptoms persist with ferritin levels >50 μg/L, gut absorption of iron is reduced, and raising iron levels becomes difficult; children with trouble sleeping should also have iron and ferritin levels checked

Caffeine: caffeine has a half-life of 5 to 7 hr and a quarter-life of 12 to 14 hr; even coffee consumed at noon can affect sleep; a cup can have ≤300 mg of caffeine

Questions and answers: probiotics — these have no proven health benefit; wearable devices — devices with oxygen saturation monitors are accurate; insomnia — in pregnant and postpartum women, insomnia may be caused by depression, anxiety, lack of time, iron deficiency, weight issues, or sleep apnea; restricting hours spent in bed is not advised for pregnant and postpartum persons as they need rest; eating patterns — these should be tailored based on frequency and length of shifts; tetrahydrocannabinol and cannabidiol — there is no data about the benefits of these for sleep