Glossary

Key terms used throughout the nuyu documentation. Terms are drawn from sleep science, neuroscience, and behavioral psychology.

A

Adenosine

A byproduct of cellular energy metabolism that accumulates in the brain during waking hours. Adenosine binds to receptors that promote sleepiness, creating what is known as sleep pressure. The longer you are awake and the more active you are, the more adenosine builds up. Caffeine works by blocking adenosine receptors without clearing the adenosine itself.

Autonomic Nervous System

The branch of the nervous system that controls involuntary functions like heart rate, digestion, and breathing. It has two main divisions: the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) systems. Sleep requires a shift toward parasympathetic dominance.

C

Chronotype

Your genetically influenced preference for earlier or later sleep timing. Chronotype exists on a spectrum from strong morning types to strong evening types, with most people falling somewhere in the middle. It is not a choice or a habit — it reflects the natural timing of your circadian clock.

Circadian Rhythm

An approximately 24-hour internal biological cycle that regulates sleep-wake timing, hormone release, body temperature, and many other physiological processes. The circadian clock is set primarily by light exposure through specialized cells in the eyes (ipRGCs).

Cortisol

A steroid hormone released by the adrenal glands in response to stress and as part of the natural circadian cycle. Cortisol peaks in the morning (the cortisol awakening response) and should decline throughout the day. Elevated evening cortisol directly antagonizes melatonin and impairs sleep onset.

D

Default Mode Network (DMN)

A network of brain regions active during self-referential thinking, mind-wandering, and rumination. The DMN is responsible for much of the "racing thoughts" experience at bedtime. Activities that produce flow states temporarily suppress DMN activity.

G

Glymphatic System

The brain's waste-clearance system, which is primarily active during deep sleep. Cerebrospinal fluid flushes through the brain tissue, removing metabolic waste products including beta-amyloid (associated with Alzheimer's disease). This system is one reason sleep deprivation has cognitive consequences beyond simply feeling tired.

H

Homeostasis

The body's tendency to maintain stable internal conditions. In the context of sleep, homeostatic sleep pressure refers to the accumulating drive to sleep that builds during waking hours (primarily through adenosine accumulation) and is discharged during sleep.

HPA Axis

The hypothalamic-pituitary-adrenal axis — the body's central stress response system. When activated, the HPA axis triggers cortisol release. Chronic HPA axis activation from sustained stress is one of the primary biological mechanisms through which stress disrupts sleep.

I

Implementation Intention

A specific plan in the format "When [situation], I will [behavior]." Research shows that implementation intentions significantly increase the likelihood of following through on intended behaviors by linking the behavior to a specific cue, reducing the decision-making required in the moment.

Interoception

The ability to sense internal body signals — heart rate, breathing, hunger, tension, fatigue. Higher interoceptive awareness allows earlier detection of stress, fatigue, and other states relevant to sleep management.

ipRGCs (Intrinsically Photosensitive Retinal Ganglion Cells)

Specialized cells in the retina that detect light for the purpose of setting the circadian clock, distinct from the rods and cones used for vision. These cells are most sensitive to blue-spectrum light and send signals directly to the suprachiasmatic nucleus (SCN).

M

Melatonin

A hormone produced by the pineal gland that signals darkness to the body and promotes sleep onset. Melatonin production begins rising in the evening (dim light melatonin onset, or DLMO) and is suppressed by light exposure, particularly blue-spectrum light. It is a timing signal, not a sedative.

N

Neuroplasticity

The brain's ability to reorganize itself by forming new neural connections throughout life. "Neurons that fire together wire together" — repeated patterns of input strengthen the corresponding neural pathways, while unused pathways weaken. This is the biological basis for why habits, once established, become automatic.

NREM Sleep

Non-rapid eye movement sleep, which includes stages N1 (light sleep), N2 (intermediate sleep with sleep spindles), and N3 (deep sleep / slow-wave sleep). NREM sleep dominates the first half of the night and is critical for physical restoration and memory consolidation.

P

Parasympathetic Nervous System

The "rest-and-digest" division of the autonomic nervous system. Parasympathetic activation slows heart rate, promotes digestion, and creates the physiological conditions necessary for sleep onset. Wind-down protocols aim to shift the balance toward parasympathetic dominance.

R

REM Sleep

Rapid eye movement sleep — the sleep stage associated with vivid dreaming, emotional memory processing, and creative problem-solving. REM sleep dominates the second half of the night, which is why cutting sleep short preferentially reduces REM and its benefits.

S

SCN (Suprachiasmatic Nucleus)

A tiny region in the hypothalamus that serves as the body's master circadian clock. The SCN receives light information from ipRGCs in the eyes and coordinates the timing of sleep, hormone release, body temperature, and other circadian processes throughout the body.

Sleep Architecture

The structure and pattern of sleep stages across the night. A typical night cycles through NREM and REM stages in approximately 90-minute cycles, with deep sleep concentrated in the early cycles and REM sleep concentrated in the later cycles.

Sleep Pressure

The homeostatic drive to sleep that accumulates during waking hours, primarily through adenosine buildup. Sleep pressure increases the longer you are awake and is discharged during sleep. It works alongside the circadian rhythm to determine when you feel sleepy.

Slow-Wave Sleep (SWS)

The deepest stage of NREM sleep (N3), characterized by large, slow brain waves. SWS is when the glymphatic system is most active, growth hormone is released, and physical restoration is at its peak. It is concentrated in the first half of the night.

Social Jet Lag

The discrepancy between your biological sleep timing (determined by chronotype) and your social sleep timing (determined by work schedules, alarms, and social obligations). Measured as the difference between your natural wake time and your alarm time across the week.

Sympathetic Nervous System

The "fight-or-flight" division of the autonomic nervous system. Sympathetic activation increases heart rate, releases stress hormones, and creates a state of alertness. Elevated sympathetic tone at bedtime is one of the primary mechanisms through which stress and anxiety impair sleep onset.

T

Two-Process Model

The foundational model of sleep regulation, describing two independent processes: Process S (homeostatic sleep pressure, driven by adenosine) and Process C (the circadian rhythm, driven by the SCN). Sleep occurs when high sleep pressure aligns with the circadian sleep window. Understanding this model explains why you can be exhausted but unable to sleep (low circadian drive) or alert despite being sleep-deprived (high circadian drive).

Z

Zeitgeber

German for "time giver" — any external cue that synchronizes the circadian clock to the 24-hour day. Light is the strongest zeitgeber, but meal timing, exercise, social interaction, and temperature also serve as time cues for the circadian system.