Baby sleep glossary
Last updated: April 2026
Short, plain-English definitions of the terms that appear across Happyly's research, FAQ and guides. Each entry cites the researchers whose work the definition rests on.
- Wake window
The stretch of time a baby can comfortably stay awake between sleeps — feeding, play, wind-down and all. Researchers call it the inter-sleep interval. Ranges from about 45 to 60 minutes at birth to 5 to 6 hours by age two. It is a range, not a target.
Source: Galland et al., 2012 (BMC Pediatrics systematic review); Weissbluth clinical data.
- Sleep pressure
The build-up of the neurochemical adenosine during wakefulness that makes a baby feel sleepy. One of the two drivers of sleep in Borbely's two-process model. The longer your baby has been awake, the higher the sleep pressure.
Source: Borbely, 1982 (two-process model of sleep regulation).
- Circadian rhythm
The internal 24-hour clock that regulates sleep, hormone release and body temperature. In newborns it is immature and not fully online until roughly 6 to 8 weeks; melatonin rhythms typically stabilise by about 3 months.
Source: Rivkees et al. on infant circadian development; Jenni & Carskadon on adolescent sleep regulation.
- Second wind
A surge of cortisol and adrenaline that hits when a baby is kept awake past their comfortable wake window. Looks like a sudden burst of energy; actually a stress response that makes settling much harder. Shorter windows for the next sleep are the fix.
Source: Weissbluth, Healthy Sleep Habits, Happy Child (clinical observations).
- Over-tired
The state a baby reaches when the wake window was too long and the second-wind cortisol response has kicked in. Cry takes on a hot, tearful quality; bouncing and rocking stop working; contact naps usually help. Not a moral failing — just biology.
Source: Weissbluth; Galland et al. on variability in normal infant sleep.
- Under-tired
The state a baby reaches when the wake window was too short. Protest without tears; baby plays, rolls, babbles in the cot. Fix: shorten the wind-down and push the next attempt 10 to 15 minutes later.
Source: Weissbluth; observed across Galland's pooled infant sleep datasets.
- Sleep regression
A temporary disruption of an established sleep pattern driven by developmental progress — motor milestones, language leaps, sleep-architecture changes. Usually resolves within 2 to 6 weeks. The 4-month and 8-to-10-month regressions are the most widely documented.
Source: Pennestri et al. 2018 (Pediatrics); Henderson on sleep consolidation; Mindell on developmental sleep transitions.
- 4-month sleep regression
The predictable shift around 3 to 5 months when sleep architecture matures from 2 stages to 4 — the baby now cycles through light and deep sleep the way adults do. Feels like a regression because previously consolidated sleep now fragments. Typically stabilises within 2 to 6 weeks.
Source: Rivkees (circadian maturation); Pennestri et al. 2018 (normal night wakings); Henderson (sleep consolidation).
- Nap transition
The weeks or months when a baby's daily nap count drops — 4+ to 3 around 3 to 4 months, 3 to 2 around 6 to 9 months, 2 to 1 around 13 to 18 months, and eventually dropping the nap entirely. Each transition typically takes 2 to 6 weeks to stabilise.
Source: Mindell & Sadeh on cross-cultural infant sleep patterns; Iglowstein et al. Zurich longitudinal cohort.
- Sleep consolidation
The gradual stretching of continuous night sleep as a baby matures. Most babies are developmentally capable of 5 to 6 hour stretches around 3 to 4 months, and 8 to 10 hour stretches by 9 to 12 months — though individual variation is wide.
Source: Henderson et al. on night sleep consolidation; Galland et al. 2012 meta-analysis.
- Bedtime routine
A predictable sequence of pre-sleep steps — bath, dim lights, feed, book, song — that primes the body for sleep. Consistency of the sequence matters more than the specific clock time. Even 3-step routines meaningfully improve sleep outcomes in controlled trials.
Source: Mindell 2009 RCT on bedtime routines; Kitsaras et al. BMC Public Health review.
- Gentle settling
Responsive sleep-support approaches — contact naps, check-and-console, pick-up / put-down, gradual retreat — that help a baby learn to fall asleep without extended unsupported crying. Happyly's coaching is grounded in this family of methods.
Source: Mindell & Owens on behavioral sleep interventions; Gradisar et al. on graduated extinction alternatives.
- Sleep-onset association
Whatever is present as the baby falls asleep — being rocked, a pacifier, a parent's presence — that they then need back in order to return to sleep during the normal brief night wakings all humans have. Neither bad nor good in itself; becomes a problem only when it is not reproducible at 2am.
Source: Ferber; Mindell on behavioral insomnia of childhood.
- Melatonin
The hormone that signals darkness to the body. Produced by the pineal gland; release is suppressed by bright light, particularly blue-spectrum light. Infants start producing their own melatonin around 8 to 12 weeks; the rhythm stabilises by ~3 months.
Source: Rivkees on infant melatonin rhythms; Borbely on circadian regulation.
- Cortisol
A stress hormone that also follows a circadian rhythm — high in the morning to wake you up, low at night. When a baby is kept awake past their window, a secondary cortisol spike fires and produces the 'second wind' effect that makes settling harder.
Source: Weissbluth on overtired-state physiology; general endocrinology literature.
- Safe sleep
The set of practices shown to reduce the risk of SIDS and sleep-related infant death — back to sleep, firm flat surface, no loose bedding, room-sharing without bed-sharing for the first 6 months. Medical, not coaching, guidance — always follow your pediatrician and your country's SIDS authority.
Source: AAP 2022 Safe Sleep policy statement; national SIDS / SUDI authorities.
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