Chapter 30: Sleep, Energy, and Peak Performance

The 3 AM Accounting

Jordan knows this particular version of consciousness too well: the 3 AM audit. Everything unresolved about the previous day — every email not sent, every decision pending, every ambiguous exchange replayed for alternate interpretations — arrives at 3 AM with unusual clarity and usually with unusual grimness.

He has had this problem since graduate school. He has, at various points, described it as anxiety (true), as overwork (also true), as the cost of caring too much about things (possibly true but not useful). What he has not, until recently, understood is that the 3 AM audit is also, partly, a neurobiological event — that the architecture of sleep itself makes the hours between 3 and 5 AM a particular kind of vulnerable window, and that the things that make it worse are not always the things he blamed.

He has been sleeping better since the wind-down practice. Four months of the shutdown ritual, the journal beside the laptop, the phone in the bag. The improvement is not dramatic — he still wakes sometimes, still lies in the particular silence of the not-quite-sleeping — but the 3 AM accounting sessions have become less frequent, and when they do arrive, they are shorter.

He wants to understand why. And he wants to understand what else he might be doing — or doing better.

This chapter is about sleep. Not as a productivity hack or a wellness prescription, but as a fundamental biological process whose design shapes nearly every aspect of waking cognitive and emotional life — and that the modern world, in almost every relevant way, is designed to undermine.

1. Sleep Architecture — What Happens When You Close Your Eyes

The Stages of Sleep

Sleep is not a single state. It is a structured cycle of neurologically distinct phases that the brain cycles through roughly every 90 minutes throughout the night. Each cycle consists of two broad categories: non-REM (NREM) sleep and REM (rapid eye movement) sleep.

NREM sleep has three stages:

• Stage 1 (light sleep): The transition from wakefulness. Muscle tone decreases; hypnic jerks may occur. Brain waves slow from alpha to theta. Easily disrupted; not restorative.

• Stage 2 (established NREM): Heart rate slows, body temperature drops. Sleep spindles (bursts of synchronized neural activity) appear; K-complexes (sharp wave patterns) occur. This stage comprises the largest proportion of total sleep time.

• Stage 3 (slow-wave sleep / deep sleep): The most physiologically restorative sleep stage. Delta waves — the slowest brain waves — dominate. Growth hormone is released. The glymphatic system — the brain's waste-clearance system — is most active. This is the stage most difficult to arouse from, and the one most directly linked to physical restoration, immune function, and declarative memory consolidation.

REM sleep: The brain is highly active — near waking levels of activity in some regions, more active than some waking states in others. The eyes move rapidly under closed eyelids. The body is essentially paralyzed (atonia), preventing the acting-out of dreams. This is when most vivid dreaming occurs. REM sleep is particularly linked to emotional memory processing, procedural skill consolidation, pattern recognition, and creative insight.

The Architecture of a Night

The cycling of these stages across a night is not uniform. The first half of the night is dominated by slow-wave sleep (NREM Stage 3), when physical restoration and declarative memory consolidation are prioritized. The second half of the night — the later hours, the hours before natural waking — is dominated by REM sleep, when emotional processing, pattern recognition, and skill consolidation peak.

This architecture has practical consequences that most people don't appreciate: sleeping six hours instead of eight does not simply remove two hours of sleep at the end of a night. It removes a disproportionate amount of REM sleep — the late-night, second-half-of-the-cycle phase. The emotional regulation, creative processing, and skill consolidation that REM provides are among the first casualties of insufficient sleep. You can cut two hours of sleep and lose more than two hours worth of restorative function.

The Glymphatic System

Discovered by Maiken Nedergaard's research group in 2012, the glymphatic system is the brain's waste-clearance mechanism: a network of channels surrounding blood vessels through which cerebrospinal fluid flows during sleep, flushing metabolic waste products out of brain tissue. Among the metabolites cleared by this system: amyloid-beta and tau proteins — the proteins implicated in Alzheimer's disease.

The glymphatic system is most active during slow-wave sleep, and nearly inactive during waking hours. The brain's daily accumulation of metabolic waste is cleared primarily at night. Chronic sleep restriction, which reduces slow-wave sleep, impairs glymphatic clearance and allows metabolic waste to accumulate.

This mechanism provides a direct biological link between chronic sleep insufficiency and long-term neurodegenerative risk — a link that is beginning to appear in longitudinal research on dementia.

2. Circadian Rhythms — The Internal Clock

What Drives Sleep Timing

Sleep timing is regulated by two interacting processes:

Process S (sleep pressure): Adenosine — a neurochemical byproduct of neural activity — accumulates in the brain across waking hours, building "sleep pressure." The longer you've been awake, the higher the adenosine concentration and the stronger the drive to sleep. Caffeine works by blocking adenosine receptors, masking the accumulated sleep pressure without eliminating it. When the caffeine wears off, the adenosine — still present — produces the characteristic "caffeine crash."

Process C (circadian rhythm): An approximately 24-hour internal biological clock, driven by the suprachiasmatic nucleus (SCN) in the hypothalamus, regulates the timing of biological processes including body temperature, cortisol release, melatonin production, and alertness. The circadian process opposes sleep pressure during the waking day and facilitates it at night — the two processes working in coordinated opposition to produce consolidated wakefulness and consolidated sleep.

The primary environmental cue that synchronizes the circadian clock to the external day-night cycle is light — specifically short-wavelength (blue) light detected by specialized photoreceptors (intrinsically photosensitive retinal ganglion cells, or ipRGCs) that project directly to the SCN. Morning light advances the clock; evening light delays it. This is the biological mechanism underlying the "sleep hygiene" recommendation to avoid screens before bed: the blue-rich light from phones, tablets, and computers suppresses melatonin production and delays the circadian signal that initiates sleep.

Chronotypes

Individual variation in circadian timing is real, measurable, and substantially genetic. Chronotype refers to the preferred timing of sleep and peak alertness:

Morning types ("larks"): natural sleep timing shifts earlier; peak alertness in the morning hours; natural waking before alarm.

Evening types ("owls"): natural sleep timing shifts later; peak alertness in the evening; difficulty waking early without sleep deprivation.

Intermediate types: the majority of adults.

Research by Till Roenneberg and colleagues finds that chronotype varies across the lifespan: children tend toward morning types, adolescents shift strongly toward evening types (a biologically driven change, not teenage rebellion), and adults gradually shift back toward earlier timing across adulthood.

The practical implication of chronotype: sleep scheduled contrary to chronotype produces worse outcomes than sleep scheduled in alignment with it. The adolescent forced to begin school at 7:30 AM is not being lazy — they are being asked to sleep and wake several hours before their biological rhythm supports it. The evening-type adult who works a 7 AM shift is chronically sleep-restricted in a way that a morning-type adult on the same schedule is not.

Social jetlag — the discrepancy between biological circadian timing and socially imposed sleep-wake timing — is associated with increased rates of obesity, metabolic syndrome, depression, and cardiovascular disease, independently of total sleep duration.

3. The Cost of Sleep Loss

What Happens When You Don't Sleep Enough

The research on sleep deprivation is among the most consistent in all of behavioral science. The effects of insufficient sleep — typically defined as less than seven hours for most adults (with normal individual variation between six and nine hours) — include:

Cognitive effects: - Reduced attention, concentration, and processing speed (effects appear within 24 hours of insufficient sleep and worsen progressively) - Impaired working memory capacity and executive function - Reduced creativity and insight — specifically, the kind of remote association and pattern recognition that depends on REM sleep - Impaired learning consolidation — new information not adequately processed into long-term memory - Slower reaction time, with effects equivalent to clinically meaningful blood alcohol levels after 20 hours without sleep

Emotional effects: - Increased emotional reactivity and decreased regulation: the amygdala response to negative emotional stimuli increases by approximately 60% after one night of sleep deprivation (Walker's research) - Reduced capacity for empathy and social functioning - Elevated levels of cortisol and other stress hormones - Increased risk of anxiety and depressive symptoms with chronic deprivation

Physical effects: - Impaired immune function (reduced natural killer cell activity, increased inflammatory markers) - Metabolic disruption (elevated blood glucose, altered appetite hormones — increased ghrelin/hunger, decreased leptin/satiety) - Cardiovascular effects (elevated blood pressure, heart rate) - Reduced pain threshold - Impaired hormonal regulation (reduced testosterone, disrupted growth hormone release)

The Underestimation Problem

One of the most important and counterintuitive findings in sleep research: people significantly underestimate the degree to which their performance is impaired by insufficient sleep. Research by Hans Van Dongen and colleagues found that subjects restricted to six hours of sleep per night for two weeks showed progressive cognitive deterioration that, by day 14, was equivalent to two full nights without sleep. Yet subjective sleepiness ratings stabilized — subjects reported feeling only slightly sleepy, despite objective performance deficits equivalent to severe sleep deprivation.

The implication: the subjective experience of being "fine" on six hours per night is not reliable evidence of adequate cognitive function. The brain adapts to a chronic sleep-deprived baseline — but the baseline itself is impaired.

This finding has significant practical importance for people who pride themselves on sleeping little: the adaptation to sleep restriction masks but does not eliminate the cognitive cost.

4. Sleep and Cognitive Performance

Memory Consolidation

Sleep plays an active role in memory consolidation — the process by which new information is stabilized in long-term memory. This is not passive storage; it involves active reprocessing and integration of new memories with existing knowledge structures.

Slow-wave sleep and declarative memory: The hippocampus, which temporarily holds newly encoded memories, "replays" them during slow-wave sleep. This replay facilitates transfer to the neocortex, where memories are integrated into long-term storage. Sleep within 24 hours of learning is particularly critical for declarative memory consolidation.

REM sleep and procedural/emotional memory: REM sleep facilitates motor skill consolidation, emotional memory processing, and the integration of new experiences with existing emotional schema. The research on skill learning is striking: musicians, athletes, and skilled practitioners who sleep immediately after practice show substantially better retention and performance improvement than those who do not.

The overnight insight effect: Wagner and colleagues (2004) demonstrated that subjects who slept between two sessions of a mathematical problem task were three times more likely to discover a hidden shortcut than subjects who stayed awake between sessions. Sleep appears to facilitate the reorganization of memory that produces insight — the "sleeping on it" phenomenon has a neurobiological basis.

Emotional Regulation and Sleep

Matthew Walker's research at UC Berkeley has been particularly influential in documenting the relationship between REM sleep and emotional regulation. The proposed mechanism: during REM sleep, the brain reactivates emotional memories in a neurochemical environment depleted of norepinephrine (a stress-related neuromodulator). This allows the emotional content of memories to be processed and integrated without the physiological stress response that the original event triggered — producing the "overnight therapy" effect.

Walker's phrase: "REM sleep takes the emotional sharp edges off the memories." This may explain why people who sleep well after traumatic events show better long-term outcomes than those who don't, and why sleep deprivation is associated with emotional hyperreactivity.

The clinical implications for therapy and mental health are substantial: many psychological difficulties — anxiety disorders, PTSD, depression — are both caused and exacerbated by sleep disruption. Treating sleep problems alongside psychological symptoms often improves both.

5. Energy Management — Beyond Sleep

The Four Energy Dimensions

Jim Loehr and Tony Schwartz, in their research on human performance with elite athletes and then with corporate executives, proposed that sustainable high performance depends on four dimensions of energy management:

Physical energy: the foundational capacity derived from sleep, nutrition, movement, and recovery. When physical energy is low, all other dimensions suffer.

Emotional energy: the quality of emotional experience — specifically, the ratio of positive to negative emotional states. Research on emotional regulation confirms that the quality of emotional experience substantially affects cognitive performance, decision quality, and relational effectiveness.

Mental energy: the capacity for focused, directed cognitive work — concentration, strategic thinking, problem-solving. This is the dimension most subject to the ultradian rhythms that will be described below.

Spiritual energy (meaning and purpose): the degree to which one's work and actions are connected to a genuine sense of purpose and values. Loehr and Schwartz found that elite performers are sustained not just by physical and mental capacity but by a clear sense of what the performance is for.

Critically, Loehr and Schwartz frame performance management as oscillation rather than endurance: the ability to fully engage and then fully disengage, to spend energy and then recover it. The pathology of modern work life is not primarily overwork but chronic low-level expenditure without adequate recovery — the slow depletion of the human performance system without the recovery cycles that would restore it.

Ultradian Rhythms

The circadian rhythm operates on a roughly 24-hour cycle. The ultradian rhythm is a shorter, roughly 90-minute cycle of alertness and recovery that operates throughout the waking day. Research by Peretz Lavie, Nathan Kleitman, and others identified that the brain cycles through periods of heightened alertness and periods of reduced alertness at approximately 90-minute intervals during waking hours.

This cycle — the Basic Rest-Activity Cycle (BRAC) — produces predictable windows of peak cognitive performance alternating with recovery troughs in which focused cognitive work is less productive. The signals of a trough include yawning, difficulty concentrating, mind-wandering, and the impulse to stretch, walk, or take a break.

Most people interpret these signals as distractions to be pushed through. The research suggests they are better understood as physiological prompts for recovery — brief periods (10–20 minutes) of low-demand activity (walking, light conversation, genuine rest) that allow the next alertness peak to arrive at full capacity.

The practical application: working in 90-minute blocks followed by genuine recovery produces more total high-quality cognitive output than working continuously across the same period. This is not a productivity trick — it is an application of the ultradian biology.

Chronobiology and Peak Performance Windows

Chronotype determines not just sleep timing but the timing of cognitive peaks across the day. Research by Carolyn Anderson, Marily Oppezzo, and Daniel Pink (among others) has documented that:

• Morning types tend to peak cognitively in the late morning (roughly 9–11 AM), experience a trough in the early afternoon (roughly 1–3 PM), and have a secondary but weaker peak in the late afternoon.

• Evening types peak later — mid-afternoon to early evening — and experience their worst cognitive performance in the morning hours when many organizations schedule their most demanding work.

• Analytical tasks (requiring focused, sequential reasoning) are best performed during peak windows.

• Insight and creative tasks (requiring diffuse, associative thinking) may be better performed during trough windows or just before sleep — when reduced executive control allows more associative, lateral thinking.

This chronobiological variation has implications for scheduling: performing demanding analytical work during your personal cognitive trough, and creative/generative work during your analytical peak, is a mismatch that reduces output quality without reducing the hours spent.

6. Sleep Hygiene — The Evidence Base

"Sleep hygiene" is an umbrella term for behavioral and environmental factors that influence sleep quality and quantity. The evidence base varies: some recommendations are very well-supported; others are more speculative.

Well-Supported Recommendations

Consistent sleep and wake times: The most consistently supported sleep hygiene recommendation. The circadian clock calibrates to regular timing; irregular timing disrupts both sleep pressure (Process S) and circadian alignment (Process C), producing "social jetlag" in the mildest form. Even one night of dramatically different timing (sleeping significantly later on weekends) can shift the circadian phase enough to impair the following week.

Light exposure: Morning bright light exposure (even overcast daylight) advances the circadian clock and improves alertness. Evening avoidance of bright (especially blue-spectrum) light — from screens and artificial lighting — preserves melatonin onset and facilitates falling asleep. The effect of screen light on melatonin suppression is dose-dependent and significantly disrupted by standard smartphone brightness settings.

Temperature: Core body temperature must drop by approximately 1–2°F to initiate and maintain sleep. A cool bedroom environment (most people's ideal is between 60–67°F / 15–19°C) facilitates this temperature drop. A warm bath or shower before bed paradoxically improves sleep by drawing blood to the skin surface and accelerating core temperature reduction after the bath.

Caffeine timing: Caffeine's half-life is 5–6 hours for most people, and a quarter-life (residual quarter of the caffeine remains active) at 10–12 hours. Coffee consumed at noon still has meaningful caffeine activity at midnight. Cutting off caffeine intake in the early afternoon substantially improves sleep quality, even in people who don't feel caffeine's effects on wakefulness.

Alcohol: Widely misunderstood as a sleep aid. Alcohol does accelerate sleep onset (due to sedative effects) but disrupts sleep architecture: it suppresses REM sleep in the first half of the night, fragments sleep in the second half (as metabolism processes the alcohol), and produces net poorer sleep quality than no alcohol, despite subjective reports that alcohol "helps sleep."

Moderately Supported

Exercise: Regular aerobic exercise is associated with improved sleep quality, reduced sleep onset latency, and increased slow-wave sleep. The timing effect is modest; the long-held caution about evening exercise interfering with sleep is not well-supported for most people, though high-intensity exercise very close to sleep can elevate core temperature and heart rate in ways that may affect sleep onset.

Napping: Research on napping is generally positive. Brief naps (10–20 minutes) improve alertness, mood, and cognitive performance without significantly impairing nighttime sleep, particularly for sleep-deprived individuals. Naps longer than 30 minutes risk entering deep slow-wave sleep, from which arousal produces significant grogginess (sleep inertia). The NASA nap study (Rosekind et al.) found that 26-minute naps improved pilot performance by 34% and alertness by 100% compared to no-nap controls.

Pre-sleep routines: Consistent pre-sleep routines appear to signal the transition to sleep and reduce time-to-fall-asleep, particularly in people with sleep-onset difficulties. The wind-down practice Jordan designed in Chapter 29 functions partly as a sleep-initiation cue.

7. Recovery as Performance Technology

The Performance-Rest Equation

The metaphor Loehr and Schwartz use is the muscle: performance is not built during exertion — it is built during recovery from exertion. Progressive overload in physical training works by stressing the muscle beyond its current capacity, then allowing adequate recovery in which the muscle rebuilds stronger. Performance without adequate recovery does not produce growth; it produces breakdown.

The same principle applies to cognitive performance. The person who works without recovery intervals, without adequate sleep, without genuine disengagement from work, is not building high performance — they are drawing down on a reserve that does not recover without rest.

Research on recovery processes by Sabine Sonnentag and colleagues identifies four dimensions of effective recovery from work demands:

Psychological detachment: mentally disengaging from work — not thinking about work problems, not reading work email, not staying in the work mental space. This is the dimension most consistently associated with reduced fatigue and improved wellbeing. Jordan's wind-down ritual is a psychological detachment intervention.

Relaxation: reduced arousal — activities that reduce physiological and psychological tension. Different for different people: meditation and walking for some; social connection, music, reading, or bathing for others.

Mastery: engaging in challenging non-work activities that produce competence and achievement experiences outside the work domain. This sounds counterintuitive but is well-supported: challenging hobbies that produce genuine skill development appear to restore depleted resources more effectively than purely passive leisure.

Control: having autonomy over the recovery period — choosing when, how, and how long to disengage from work. Enforced leisure is less restorative than chosen leisure.

Weekend and Vacation Recovery

Research on recovery across longer periods (weekends, vacations) produces an initially depressing finding: the recovery benefits of a vacation largely dissipate within a week of return. The vacation itself produces significant wellbeing improvements; the return to unchanged work conditions reverses them.

What predicts more sustained recovery effects: the quality of recovery during the vacation, not the vacation itself. Vacations in which work is mentally and communicatively present (checking email, taking calls, ruminating about work) produce significantly lower recovery than vacations characterized by full psychological detachment.

And within-week recovery — the daily disengagement practices, the sleep quality, the brief recovery intervals within workdays — predicts performance and wellbeing at least as much as annual vacation time. The sustainable performance system is one that includes recovery at every timescale: within the workday (ultradian recovery intervals), overnight (adequate sleep), weekly (genuine weekend recovery), and occasionally across longer periods.

8. Sleep Disorders and When to Seek Help

Sleep problems are the most common untreated health condition in the developed world. The most prevalent:

Insomnia: difficulty falling asleep, staying asleep, or waking too early, with daytime impairment. Approximately 10–15% of adults meet diagnostic criteria for chronic insomnia. Cognitive Behavioral Therapy for Insomnia (CBT-I) is the first-line, evidence-based treatment — superior to sleep medication in long-term outcomes, without the side effects or dependency risk. CBT-I addresses the thoughts, behaviors, and patterns that perpetuate insomnia independently of the original causes.

Obstructive sleep apnea (OSA): repeated partial or complete airway obstruction during sleep, causing fragmented sleep and oxygen desaturation. Affects approximately 15–30% of adults, with higher rates in middle-aged men and older adults. Dramatically underdiagnosed. Strongly associated with cardiovascular disease, metabolic syndrome, cognitive impairment, and depression. Treatment (most often continuous positive airway pressure, CPAP) produces substantial improvements in cognitive function, mood, and cardiovascular risk.

Restless legs syndrome and periodic limb movement disorder: urge to move the legs, particularly at night, disrupting sleep onset and maintenance.

Circadian rhythm disorders: advanced sleep phase (sleeping and waking very early), delayed sleep phase (sleeping and waking very late), and shift work disorder. These are not willpower problems; they reflect genuine biological timing dysregulation requiring chronobiological interventions.

The important clinical message: persistent sleep problems that don't respond to sleep hygiene measures should be evaluated professionally, not managed with alcohol, sleeping pills as the primary treatment, or chronic sleep restriction. CBT-I is available, effective, and increasingly accessible (including digitally).

From the Field — Dr. Reyes on Sleep and Psychological Treatment

"In my clinical training, sleep was treated as a symptom. If a patient presented with depression or anxiety, the sleep disturbance was downstream of the primary problem. Treat the depression; the sleep will follow.

My experience across three decades told a different story. The relationship between sleep and psychological wellbeing is bidirectional, and frequently the sleep problem came first or was independently maintaining the psychological problem, even when the original precipitant was something else entirely. I began treating sleep problems directly — not as a separate track, but as central to the primary treatment.

The patients who improved fastest were, disproportionately, the ones who had made some progress on their sleep. Not because sleep was magic, but because the emotional regulation, cognitive clarity, and resilience that adequate sleep provides made the therapeutic work more accessible. You cannot do the emotional work of therapy effectively on two or three hours a night.

The other thing I learned: people substantially overestimate their tolerance for sleep restriction and substantially underestimate the emotional toll of it. The patient who said 'I only need six hours, I've always been like this' — I learned to ask, gently, what their emotional life looked like and when they last felt fully rested. The answers were usually instructive.

Sleep is not a luxury. It is the maintenance schedule for the human cognitive and emotional system. Missing it doesn't mean the system stops running. It means the system runs degraded, in ways that are mostly invisible from the inside."

Research Spotlight: The Amygdala and Sleep Deprivation

Matthew Walker and colleagues at UC Berkeley conducted a study (Yoo et al., 2007, Current Biology) comparing the amygdala responses of well-rested and sleep-deprived subjects to emotionally negative images.

Sleep-deprived subjects showed approximately 60% greater amygdala activation to the same emotional stimuli compared to well-rested controls. Additionally, sleep-deprived subjects showed a decoupling of the prefrontal cortex-amygdala circuit — the regulatory connection between the "thinking brain" and the emotional response system was weakened. The amygdala was firing more, and the prefrontal regulation of that response was reduced.

The research team also found that sleep-deprived subjects showed a "loss of emotional neutrality" — they rated a much broader range of images as threatening or negative, including images that well-rested subjects rated as neutral. Essentially, sleep deprivation produced a systematic bias toward negative emotional interpretation.

The implication is significant for everyday life: the emotional reactivity, irritability, and catastrophizing that people often attribute to stress or personality are, in part, neurobiologically predictable consequences of insufficient sleep. And the regulatory capacity — the ability to think before reacting — is specifically impaired by the prefrontal-amygdala decoupling.

Adequate sleep is not just restorative for cognitive function. It is the biological foundation of emotional self-regulation.

Chapter Summary

Sleep is not a passive state of inactivity. It is an active biological process — cycling through distinct architecturally organized phases — that accomplishes critical functions unavailable during waking: memory consolidation and integration, emotional processing, metabolic waste clearance (glymphatic system), immune maintenance, and hormonal regulation.

Insufficient sleep produces impairments across cognitive, emotional, and physical domains that are substantially more severe than subjectively apparent. The cultural norm of sleeping less to accomplish more produces a chronic performance deficit that compounds across days and weeks.

The practical framework:

• Sleep architecture: protect the full night, especially the later REM-rich hours
• Circadian alignment: consistent timing, morning light, evening darkness
• Chronotype: schedule demanding work during your personal peak; give yourself permission to sleep in alignment with your biology
• Ultradian rhythm: work in roughly 90-minute blocks with genuine recovery intervals
• Recovery: detachment, relaxation, mastery, and control across all timescales
• Sleep hygiene: consistent timing, cool temperature, caffeine cutoff in early afternoon, alcohol avoidance as a sleep aid
• Help-seeking: CBT-I for insomnia; evaluation for sleep apnea if snoring, fatigue, and mood problems are present

Key Terms

NREM sleep — non-rapid eye movement sleep; three stages from light (Stage 1) to deep slow-wave sleep (Stage 3); dominates the first half of the night.

REM sleep — rapid eye movement sleep; high brain activity, muscle atonia, vivid dreaming; dominates the second half of the night; critical for emotional processing, procedural skill consolidation, and creative integration.

Slow-wave sleep — NREM Stage 3; the most physiologically restorative sleep stage; associated with glymphatic clearance, growth hormone release, and declarative memory consolidation.

Glymphatic system — the brain's cerebrospinal fluid-based waste-clearance mechanism; most active during slow-wave sleep; clears metabolic waste including amyloid-beta and tau proteins.

Process S (sleep pressure) — the adenosine-based accumulation of sleep drive across waking hours; caffeine blocks adenosine receptors, temporarily masking sleep pressure.

Process C (circadian rhythm) — the approximately 24-hour internal biological clock, driven by the SCN; regulates the timing of alertness and sleep through interaction with light, melatonin, and cortisol.

Chronotype — individual variation in the preferred timing of sleep and peak alertness; substantially genetic; ranges from morning type ("lark") to evening type ("owl").

Social jetlag — the discrepancy between biological circadian timing and socially imposed sleep-wake timing; associated with metabolic, cardiovascular, and psychological health consequences.

Ultradian rhythm / BRAC — the Basic Rest-Activity Cycle; roughly 90-minute cycles of alertness and recovery during the waking day; suggests 90-minute work blocks followed by genuine recovery.

Cognitive Behavioral Therapy for Insomnia (CBT-I) — the evidence-based first-line treatment for chronic insomnia; addresses the thoughts, behaviors, and patterns perpetuating insomnia; superior to sleep medication in long-term outcomes.

Obstructive sleep apnea (OSA) — repeated airway obstruction during sleep, producing fragmented sleep and oxygen desaturation; dramatically underdiagnosed; strongly associated with cognitive impairment, cardiovascular risk, and depression.

Psychological detachment — the recovery dimension involving genuine mental disengagement from work; the dimension most consistently associated with reduced fatigue and improved wellbeing.