The sleep words, defined.

Plain-language definitions of the sleep-science terms Nightpenny uses, plus the app's own vocabulary (Pennie, the Sunday ritual, the 14-night balance, the trend signal). Where a term connects to the math the app actually runs, the entry links to the methodology page.

Not a medical reference. For sleep problems, see a healthcare provider. Nightpenny is not a diagnostic tool.

14-night balance (Nightpenny)

The headline number on Nightpenny's gauge — the running sum of the last 14 daily deltas (each positive delta past the first hour is discounted to 60%). The 14-night window comes from Van Dongen et al. 2003. Why 14 nights, and the recovery formula →

Biphasic sleep

A sleep pattern split across two periods in a 24-hour cycle, typically a long night plus an afternoon nap (the classic siesta). Contrasts with monophasic (one block) and polyphasic (many blocks).

Chronotype

Your natural preference for being awake or asleep at certain times — colloquially "morning person" versus "night owl." Driven largely by genetics and circadian phase, modulated by light exposure and age.

Circadian rhythm

The roughly 24-hour internal clock that regulates alertness, body temperature, hormone release, and sleep propensity. Entrained primarily by light hitting the suprachiasmatic nucleus.

Daily delta (Nightpenny)

The night's actual sleep duration minus your target. Positive means a deposit; negative means a withdrawal; zero is a flat night. The building block for Nightpenny's 14-night balance. How Pennie computes the delta →

Forecast heatmap (Nightpenny)

A 7-day × 24-hour grid in the app's Forecast tab. Cells are shaded by a likelihood score combining the circadian post-lunch dip, the pre-bed wind-down, and weekdays that have historically been worse for you. Patterns, not predictions. How the heatmap is built →

Homeostatic sleep pressure

The drive to sleep that accumulates with every waking hour and dissipates during sleep. Borbély labelled this Process S in his 1982 two-process model. Process S in the methodology →

Insomnia

Persistent difficulty falling asleep, staying asleep, or waking too early despite adequate opportunity. The gold-standard treatment is CBT-I (cognitive-behavioural therapy for insomnia), not sleep trackers. Nightpenny is not a clinician.

Jet lag

The temporary mismatch between your circadian phase and the local clock after rapid travel across time zones. Symptoms usually resolve in one day per time zone crossed, faster eastward than westward.

Ledger (Nightpenny)

The 14-night log in the app — one row per night, showing duration, target, delta, and the source (Apple Watch, iPhone bedtime, manual entry). The data behind the balance.

Manual entry (Nightpenny)

Logging a night's sleep duration by hand when HealthKit missed it or you slept somewhere your watch didn't follow you. Available on every Nightpenny tier including free — it's a data-quality lever, not a paid feature.

Melatonin

A hormone secreted by the pineal gland in response to darkness; rising melatonin levels in the evening promote sleep onset. Bright light at night (including phone screens) suppresses melatonin release.

Microsleep

Brief, involuntary episodes of sleep lasting a fraction of a second to about 30 seconds, often without awareness. Common consequence of significant sleep debt and a major contributor to drowsy-driving incidents.

Monophasic sleep

A single consolidated sleep period per 24 hours — the dominant pattern in modern industrial societies. Contrasts with biphasic (one long sleep plus a nap) and polyphasic (multiple short sleeps).

Pennie (Nightpenny)

The copper piggy-bank mascot of Nightpenny. Pennie keeps your sleep ledger, totals the 14-night balance, and walks you through the Sunday ritual. The face you see on the home gauge.

Process C

The circadian alerting signal in Borbély's two-process model — a roughly 24-hour oscillation independent of prior sleep/wake history. Its trough is the post-lunch dip; its peak holds you alert in the evening. The two-process model →

Process S

The homeostatic component of Borbély's two-process model. Rises with every hour of wakefulness, dissipates during sleep. The longer you've been awake, the higher S climbs. The two-process model →

REM sleep

Rapid Eye Movement sleep — the stage most associated with vivid dreaming, characterised by rapid eye movements, near-paralysis of voluntary muscles, and brain activity close to waking. Dominant in the second half of the night.

Repayment plan (Nightpenny)

A multi-night schedule the app generates from the Sunday ritual — small earlier bedtimes spread across the week to bring a negative balance back toward zero without a single brutal recovery night.

Sleep architecture

The structural pattern of sleep stages across the night — cycling through N1, N2, N3 (slow-wave) and REM, roughly every 90 minutes, with N3 dominating the first half and REM the second.

Sleep cycle

One full progression through the sleep stages (typically N1 → N2 → N3 → N2 → REM), lasting roughly 90 minutes. Adults complete four to six cycles per night.

Sleep debt

The cumulative shortfall between sleep you needed and sleep you got, summed over a rolling window. Nightpenny visualises this as the running 14-night balance — short nights withdraw, good nights deposit. How Nightpenny calculates it →

Sleep efficiency

Total time asleep divided by total time in bed, expressed as a percentage. 85% or higher is generally considered healthy; persistently lower values can indicate fragmented sleep or insomnia.

Sleep fragmentation

Frequent brief awakenings (often unremembered) that break up sleep into shorter pieces, reducing restorative value even when total time-in-bed looks adequate. Common causes include sleep apnea and noise.

Sleep inertia

The grogginess and impaired performance immediately after waking, especially from deep stages. Typically resolves within 15–30 minutes; can last longer if you wake from N3 slow-wave sleep.

Sleep latency

The time it takes to fall asleep after turning the lights out. Typical values are five to twenty minutes; falling asleep in under five minutes consistently can indicate significant sleep deprivation.

Sleep restriction

Deliberately limiting time in bed below normal sleep need — used clinically as part of CBT-I to consolidate sleep, and used in research (e.g. Van Dongen 2003) to study the dose-response cost of insufficient sleep.

Slow-wave sleep (N3 / SWS)

The deepest stage of non-REM sleep, named for the high-amplitude delta waves dominant in the EEG. Associated with physical recovery, immune function, and memory consolidation; dominant in the first half of the night.

Social jet lag

The chronic mismatch between your biological sleep schedule and the schedule imposed by work or school. Quantified as the difference between mid-sleep on workdays and free days. Most pronounced in late chronotypes.

Sunday ritual (Nightpenny)

Nightpenny's weekly three-minute review. Pennie summarises the week, you pick a pace, and the app writes earlier bedtimes for the coming week into your iOS Calendar.

Trend (Nightpenny)

A 7-night directional signal — repaying, accruing, or steady — alongside the 14-night balance. A −4h balance with a "repaying" trend tells a different story than a −4h balance with an "accruing" one. The thresholds →

Two-process model

Alexander Borbély's 1982 model of sleep regulation — Process S (homeostatic sleep pressure) and Process C (circadian alerting signal) operating independently and combining to set sleep propensity. Reappraised by Borbély, Daan, Wirz-Justice and Deboer in 2016. How it shapes the heatmap →

Wakefulness after sleep onset (WASO)

Total time spent awake during the sleep period, between first falling asleep and final waking. A standard polysomnography metric; lower WASO indicates more consolidated sleep.

Missing a term? Email contact@nightpenny.com. The glossary is curated, not exhaustive — additions land in the next release after they're flagged.